Thanks for the hint. It was the name of the webmail services with an umlaut. Why does this but only when a domain has been the case is beyond me. Also the Plesk this after updating to version 11.5 can no longer weak. Before the update it worked, and the software allows saving to.
Solution...
When I say "email" Click in the administration of a Abonoments on the tab, the following error message appears.
However, this error only occurs with a single Abonoment. How can I fix this?
[18-Jun-2014 23:39:06 Europe/Berlin] PleskMainDBException: MySQL query failed: Illegal mix of...