I could not find any other explanation except that at some point some converter was using the wrong time zone.
In the end I've fixed the problem setting a default time zone before the execution of the test.
After a quick search on the web this seems a good thing to do anyway.

By the way, before I've changed it, this tests used to fail sporadically with the ehcache too.
There was an obscure comment about it in the code with no explanation given, maybe you know something about it.