Redmine

Hello all,

when I change the field "Assigned to" in a ticket to another user and want to save it, a new page opens with the following error message:

"Internal error
An error occurred on the page you were trying to access.
If you continue to experience problems please contact your Redmine administrator for assistance.

If you are the Redmine administrator, check your log files for details about the error."

The ticket can then only be used further if I re-insert the original user in the "Assigned to" field. How can I solve this problem?