After updating the author instance (new war deployed) and the public instance (updated classes) the activation doesn't work any longer.

it's the same issue like described here:
http://forum.magnolia-cms.com/forum/thread.html?threadId=492f1187-b21b-4039-9355-2ce9145a7dc3

Here is the relevant error log:

2013-07-12 10:15:55,593 ERROR fo.magnolia.module.exchangesimple.SimpleSyndicator: java.lang.NullPointerException
info.magnolia.cms.exchange.ExchangeException: java.lang.NullPointerException
at info.magnolia.module.exchangesimple.BaseSyndicatorImpl.activate(BaseSyndicatorImpl.java:443)
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)
at java.lang.Thread.run(Thread.java:679)
Caused by: java.lang.NullPointerException
at java.math.BigInteger.subtract(BigInteger.java:1105)
at org.bouncycastle.crypto.engines.RSABlindedEngine.processBlock(Unknown Source)
at org.bouncycastle.jce.provider.JCERSACipher.engineDoFinal(Unknown Source)
at javax.crypto.Cipher.doFinal(Cipher.java:1813)
at info.magnolia.cms.security.SecurityUtil.encrypt(SecurityUtil.java:225)
at info.magnolia.cms.security.SecurityUtil.encrypt(SecurityUtil.java:186)
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)
... 4 more