Thanks for the help!
You are right, I tried to run a script in a folder, but this worked without any problems before the update.
Now I moved pipista into site-packages and the import from the script in the folder worked, but when the script tries to import a module from the pypi-modules folder I got the same error.
I moved the pipy-modules folder into the site-packages folder but this doesn't change anything ... but wait I forgot that that site-packages folder has to be reload or something like that ... I will try now ...
Edit: After reloading everything in the site-packages folder importing from pypi-modules is working again.
Thanks for the help!