After a restart the activation works again.

java.lang.SecurityException: Failed to retrieve private key. Please make sure the key is located in WEB-INF/config/default/keypair.properties
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.activate(BaseSyndicatorImpl.java:358)
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.activate(BaseSyndicatorImpl.java:282)
at info.magnolia.module.admininterface.commands.ActivationCommand.activateUpdate(ActivationCommand.java:123)
at info.magnolia.module.admininterface.commands.ActivationCommand.execute(ActivationCommand.java:94)
at info.magnolia.commands.MgnlCommand.executePooledOrSynchronized(MgnlCommand.java:174)
at info.magnolia.commands.MgnlCommand.execute(MgnlCommand.java:161)
at info.magnolia.commands.DelegateCommand.execute(DelegateCommand.java:62)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at info.magnolia.cms.servlets.CommandBasedMVCServletHandler.execute(CommandBasedMVCServletHandler.java:96)
at info.magnolia.cms.servlets.MVCServlet.doPost(MVCServlet.java:125)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at info.magnolia.cms.filters.ServletDispatchingFilter.doFilter(ServletDispatchingFilter.java:123)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:85)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:85)
at info.magnolia.cms.filters.CompositeFilter.doFilter(CompositeFilter.java:67)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.VirtualUriFilter.doFilter(VirtualUriFilter.java:70)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.module.cache.executor.Bypass.processCacheRequest(Bypass.java:58)
at info.magnolia.module.cache.executor.CompositeExecutor.processCacheRequest(CompositeExecutor.java:66)
at info.magnolia.module.cache.filter.CacheFilter.doFilter(CacheFilter.java:153)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.i18n.I18nContentSupportFilter.doFilter(I18nContentSupportFilter.java:76)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.RangeSupportFilter.doFilter(RangeSupportFilter.java:84)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.security.BaseSecurityFilter.doFilter(BaseSecurityFilter.java:60)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.security.SecurityCallbackFilter.doFilter(SecurityCallbackFilter.java:86)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.security.LogoutFilter.doFilter(LogoutFilter.java:93)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.module.templatingkit.filters.SiteMergeFilter.doFilter(SiteMergeFilter.java:99)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.MultiChannelFilter.doFilter(MultiChannelFilter.java:83)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.module.cache.filter.GZipFilter.doFilter(GZipFilter.java:75)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:85)
at info.magnolia.cms.security.auth.login.LoginFilter.doFilter(LoginFilter.java:93)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:85)
at info.magnolia.cms.filters.CosMultipartRequestFilter.doFilter(CosMultipartRequestFilter.java:91)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.module.devicedetection.filter.DeviceDetectionFilter.doFilter(DeviceDetectionFilter.java:73)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:61)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.ContentTypeFilter.doFilter(ContentTypeFilter.java:102)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.ContextFilter.doFilter(ContextFilter.java:131)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:83)
at info.magnolia.cms.filters.CompositeFilter.doFilter(CompositeFilter.java:67)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:91)
at info.magnolia.cms.filters.SafeDestroyMgnlFilterWrapper.doFilter(SafeDestroyMgnlFilterWrapper.java:108)
at info.magnolia.cms.filters.MgnlFilterDispatcher.doDispatch(MgnlFilterDispatcher.java:67)
at info.magnolia.cms.filters.MgnlMainFilter.doFilter(MgnlMainFilter.java:108)
at info.magnolia.cms.filters.MgnlMainFilter.doFilter(MgnlMainFilter.java:94)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
at org.apache.coyote.ajp.AjpProcessor.process(AjpProcessor.java:200)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Caused by: info.magnolia.cms.exchange.ExchangeException: 1 error detected:
java.lang.SecurityException: Failed to retrieve private key. Please make sure the key is located in WEB-INF/config/default/keypair.properties
at info.magnolia.module.exchangesimple.SimpleSyndicator.handleErrors(SimpleSyndicator.java:95)
at info.magnolia.module.exchangesimple.SimpleSyndicator.activate(SimpleSyndicator.java:86)
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.activate(BaseSyndicatorImpl.java:322)
... 89 more
Caused by: java.lang.SecurityException: Failed to retrieve private key. Please make sure the key is located in WEB-INF/config/default/keypair.properties
at info.magnolia.cms.security.SecurityUtil.getPrivateKey(SecurityUtil.java:263)
at info.magnolia.cms.security.SecurityUtil.encrypt(SecurityUtil.java:185)
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.addActivationHeaders(BaseSyndicatorImpl.java:591)
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.transportActivatedData(BaseSyndicatorImpl.java:454)
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.activate(BaseSyndicatorImpl.java:405)
at info.magnolia.module.exchangesimple.SimpleSyndicator$2.runTask(SimpleSyndicator.java:103)
at info.magnolia.module.exchangesimple.ExchangeTask.run(ExchangeTask.java:75)
at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(Unknown Source)
... 1 more
Caused by: java.io.FileNotFoundException: WEB-INF/config/default/keypair.properties (No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:120)
at java.io.FileInputStream.<init>(FileInputStream.java:79)
at info.magnolia.cms.security.SecurityUtil.getPrivateKey(SecurityUtil.java:258)
... 8 more