Hi Felix,
Did the initial error occur the first time you tried to push files to gitlab?
I ask because I was having very similar issues after trying to push commits to a project that I had already created. My mistake was that I had multiple accounts made on gitlab and my username and password were set to the wrong account. I was getting errors like this:
cmdline: git push -u origin master
stderr: ‘remote: HTTP Basic: Access denied
fatal: Authentication failed for ‘https://gitlab.pavlovia.org/unagi_pie/new-ldt.git/ ’’
I was able to fix this by going to the windows ‘credential manager’ in the control panel. If you go there, you should be able to check the state of your username and password for gitlab.pavlovia.org. That might provide some useful info.