freemarker can't load tag libraries when magnolia (magnolia/trunk) is run inside eclipse. The magnolia webapp is run from the eclipse workspace, without copying resources to a standard webapp layout (WEB-INF/lib doesn't exists and taglib modules are not jarred).
Unfortunately freemarker looks for taglib in a WEB-INF/lib folder explicitly and dies with a NPE if the folder is missing:

Caused by: java.lang.NullPointerException
at freemarker.ext.jsp.TaglibFactory.getLocationsFromLibJars(TaglibFactory.java:290)
at freemarker.ext.jsp.TaglibFactory.getMoreTaglibLocations(TaglibFactory.java:206)
at freemarker.ext.jsp.TaglibFactory.get(TaglibFactory.java:163)

.. or simply complains about the missing taglib if an empty WEB-INF/lib directory is created:

freemarker.template.TemplateModelException: Could not find webapp resource /cms-taglib
at freemarker.ext.jsp.TaglibFactory.loadTaglib(TaglibFactory.java:432)
at freemarker.ext.jsp.TaglibFactory.access$300(TaglibFactory.java:106)
at freemarker.ext.jsp.TaglibFactory$Taglib.load(TaglibFactory.java:414)

(tried by simply adding the demo-project dependency to the empty webapp)

unfortunately really annoying during development...