P6 Client version 16.1 issue with assigning resources

I'm working with a client who is using P6 Client version 16.1 (stand alone install), and is having an issue assigning resources. He had been able to assign resources, but recently the assign resource window has stopped opening in his P6. He has closed the project and P6 numerous times, but still can't open the assign window. It doesn't matter if he uses the Add Resource button in the Resource tab or if he tries to click the resource icon in the command bar the window will not open. Has anyone else using 16.1 with the client come across this issue?

Erin