1. You can safely ignore the messages, the viruses should be disinfected correctly even though this is logged.
2. It is an interesting implication of the way the avast unpacker now works. That is, if an infected file is the only file in the archive, the archive is deleted in its entirety (yes, it's trying to be smart and not leave any empty archives on the disk). However, it seems that the Exchange plugin somehow doesn't count with that, and logs this warning (error code 00000002 means "file not found" which makes perfect sense as the file was deleted). We will fix this in the next avast program update ("fix" means suppress logging of these messages).
Thanks
Vlk