What can cause a HV000028 validation exception?

From your stack trace the validation failed on some NullPointerException, but it's on the last line. You should have posted a full stack trace.

Also please note, it's not a Hibernate (an ORM) that has caused the exception, but Hibernate Validator, which is a completely different thing.

This validator has a series of validations executed on input object that is called from wicket, see: ch.lmv.ulm.web.page.template.BasePanel.doCompleteJSR303Validation.

Now the bad part is that your logging system probably is not correctly configured. It's hard to say what exactly happens with your logging system, because you do not supply any details about it.

In a correct configuration, the exception with the stack-trace should be printed as a multiline string (one single INFO message) and not as a series of Messages (you have INFO on every single line, and it's wrong).

The correct way of calling the log (for example in slf4j framework) should be:

try {
   ... execute validation code
}catch (<SomeKindOfValidationExceptionYouExpectToGet> ex) {
   logger.error("Failed to validate <or better message>", ex); 

Note, that you pass an exception as an additional parameter here.