Mail.app got stuck and doesn't sync anymore

Ok, I'll leave all my comments and the frustration out of this post. As many of other people around I can't get Mail.app sync anymore with GMail server.

Unsucessfully I tried to remove and create the account, including deleting the account.plist file in the folder (just in case).

Looking into the console file, I spot this message, that doesn't offer any clue onf what's wrong.

Mail[7196]: *** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]

or

Mail[7196]: An exception occurred during invocation of -[IMAPMailboxSyncEngine _goWithMessagesIfNeeded:]

Anybody knows what they are? Any idea how to get this fixed?