Hi Folks,
uninstalling and reinstalling several times does not change anything on EpicInstaller Build 12.1.7-enterprise( installation tried out on 18.08.2021).
What worked for me was a click on the install button for Twinmotion. After a few seconds, the Install Button for the Engine was reactive again and I could proceed.
You might have to delete the original Program Files folder for the concerned UE version, if as in my case you had a working version already installed.
Well, good luck.