Regarding this message:

I forgot how to fix this (I am working with the java version)
What I have tried is deleting the xif + subfolder on my drive.

Any tips how to solve it?

Solved by Technical Support:
there is a lock file created in "C:\Users\<your username>\AppData\Local\Temp" for each open project.

Please close all your integration studio instances and delete the *.lock files from this folder then try to open the extension again.