Just to see if anyone else has the same problem:

after applying the latest java updates on Snow Leopard (done on the 9th of december) I realized lot of instabilities with the MAC Client. It crashes if I open an pdf attachement (not when I open it, but if I close it Client will crash). Also multi select of mails makes the client crashing.

On another System we realized that setting up a cashing mailbox ends up with D109 errors on every item and the caching mailbox was not functional (even if it states that initial sync was successful).