Problem with user account

Hello,

We are running SD 7.1 SP1 and recently we have problems with one user account. At first glance everything seems OK with this account (as seen in admin - users - accounts - manage users) but when assigning a ticket to this user, the field "assigned to" stays empty. (See screenshot)

It is perfectly possible to select this user when assigning, and in the History you can see it has been assigned to this user, but it does not appear in his task list and as said the assigned to box is empty.

I hope I made myself clear, if not I will be happy to provide details you might need.

Has anyone had problems like this and how did you solve this?

Thanks for your help.

Koen