Can't assign members to tasks in Planner

Suddenly on my Tasks by Planner and To Do page, I can't assign any members to the tasks. When I click Assign icon, I get a pop up, but all the members' names are blank. If I search for a name, the pop up box just closes. I've tried shutting down Teams in the Task Manager and rebooting my PC, but the problem still persists. It was working fine up until yesterday.