If ETS does not start or it starts but it's not usable, then it could be that the 'Cache' folder contains invalid and/or inconsistent data.
How to identify?
If you open with the notepad the 'ETS.log' from the directory 'C:\Users\YourUserName\AppData\Local\KNX\ETS5\Log' and you find an error similar to that one:
ERROR System.Windows.Application - System.Windows.Markup.XamlParseException:
Die Datei "C:\Windows\system32\PBMSfvRFDqA" konnte nicht gefunden werden. --->
System.IO.FileNotFoundException: Die Datei "C:\Windows\system32\PBMSfvRFDqA"
konnte nicht gefunden werden.
How to solve?
Clear the contents from the ETS5 cache folder in the directory: C:\Users\Username\AppData\Local\KNX\ETS5\Cache
In case you cannot find this folder, it may be because it is hidden. See here to read how to make it visible.
Then, start ETS again.