Thanks wroot,
so its all related...
The "funny" thing is that this did not happen all the time.
We had several restarts before and everything was fine.
The "lost cache" happened only when there were like 7k messages stored in the cache...so, it all seems to come down to broken cache behavior...propably...
I am not an expert at openfire, but we use openfire as a backend for our chatting solution and I am in charge of handling this backend.