Crash then will not terminate without reboot

I’m having an issue where UE5 will freeze. I can then only close UE5 by rebooting the PC. I cannot terminate the program either in task manager or by using taskkill in cmd or Powershell. Rebooting is the only way.
It can happen at any point. The last was after I had a new level that I had then duplicated a smallish number of megascans at medium download. I had played then moved then played again. went to save and freeze. Never had the issue with anything else.
Any help would be greatly appreciated.

As a test I have been using UE5 in Sandboxie-Plus. Seems to be running great. I’ve had one freeze but I just terminated the sandbox and its all good again. Strange.