Please see https://bugzilla.redhat.com/show_bug.cgi?id=1018227 for more details, especially Martins comment: "Yeah, the Microsoft SQL server does not seem to support UTF-8. It means that for string-based cache stores only "String" can be used as a key. The other types (coverted with Key2StringMapper) cannot. AFAIK, it also means that string-based cache store cannot be used at all if data is stored via HotRod. That's because HotRod does not send plain strings."
Sorry, this was a misunderstanding. *** This bug has been marked as a duplicate of bug 1018227 ***