Hello Lajos Illich ,

Please try first resetting Windows Layout as the error appears to reference the UI

Now close Studio and ensure everything is updated (OS/drivers etc)

https://community.sdl.com/product-groups/translationproductivity/w/customer-experience/3392/01-2-trouble-shooting-unexplained-errors-and-unwanted-behaviour-in-trados-studio-multiterm

(Parts 1&2)

Its also worth checking that you have the latest .net version installed.

Now restart your machine and Studio and check that Studio is on the latest update.

Hello Steven,

Thanks for your reply. I tried what you suggested above. Alas, all is still the same.

I have the same problem. I upgraded a lot of TMs without any problen and now I get this error message, too. Does anyone have a solution for the problem?

Hi Lajos Illich, Doctec B.V., 
Try to install this plugin: https://appstore.sdl.com/language/app/sdl-trados-compatibility-and-migration-power-pack/1101/
It worked for me. 
This feature was an integral part of 2019, but starting from 2021, it was removed. That is why you experience this issue after your uprade from 2019 to 2021.

If your version didn't change, but the error message started popping up just out of the blue, probably you need to install NET Framework 4.8.0. I did both: installed NET Framework 4.8.0 first and then the compatibility pack. I tried to upgrade a TM after the first step (once I installed NET Framework 4.8.0) to no effect. Then I installed the compatibility pack and rebooted my computer. However I am not sure whether it was NET Framework or compatibility pack that helped because I did not reboot after I installed NET Framework. Maybe it could work if I rebooted as soon as I installed NET Framework.