You tried the working experiment. 
The first link (https://run.pavlovia.org/D3njo/erlernen-artifizieller-gleichungen_1/html/) is the paradigm with the error.
Emptying the cache also wasn’t the solution.
The actual solution was: Deleting the html-folder, the .git-folder and the .gitignore-file, renaming the folder, the psyexp-file and changing the title of the experiment and then uploading it as a new project.
I actually have tried the exact same thing before and the error persisted. But now it’s working and I’ve compared every file of the old and the new upload in PyCharm, but there are now differences (except the experiment-name), so I think it is some server related issue.
I’ll leave the non-working version online so @jon can check if or what’s the problem with pavlovia.