I forgot to mention that the _pattern property of javax.faces.convert.DateTimeConverter is an empty string, hence getAsString() which returns prepareDateFormat().format(value) returns an empty string.
This is because of prepareDateFormat() returning new SimpleDateFormat(_pattern, getLocale()).
I messed up my tests :-(
More testing showed that the problem is not related to tomcat. I reposted to the right forum (I hope).
Sorry for the trouble! (But nobody seems to bother about this topic anyway)