Symptom: When attempting to add a new mail account you receive the following error message: Unable update mailbox: mailmng failed: Empty error message from utility
Solution: The error is caused by a security update released in Ubuntu for mysql 5.1.66, there is currently no Plesk fix for this error as of Nov. 13th, 2012. In order to resolve the error you must roll back 5.1.66 to 5.1.63.
Instructions can be found here for the temporary fix: Parallels Forum
Note: If Ubuntu 10.04 LTS is set to automatically install updates, every time it checks for updates it will patch 5.1.63 to 5.1.66 again.