The new evaluation will try to fix missing namespace bindings for PropertyNames in expressions and filters. Such broken bindings can be hard to find, because even though the schemas require you to use qualified names/xpaths, normal schema validators will not find these errors.
One nice (and intended) side effect is that SLD files used to configure GeoServer can now be used without modification (unless some special GeoServer constructs are used).
Since it is still desirable to write valid documents, you can set a class to DEBUG in your log4j.configuration to see what PropertyNames are being repaired:
log4j.logger.org.deegree.filter.Filters = DEBUG