Sorry, I should have found the solution sooner: but going back to Widevine 4.10.2252.5 solved the problem for me. You can go back with the Inputstream helper addon, or this way:
https://www.matthuisman.nz/2020/02/slyguy…repository.html > install Slyguy Common. And then go to Services > Slyguy Common > Advanced > Re(install) Widevine CDM and choose 4.10.2252.5