Thank you - it works now.
After checking the logs i saw that the difference must have been the collectstatics. In an earlier (failed) build, the logs told me to disable collectstatic, which i did back then - without knowing why... Afterwards deploying worked, but the app didnt run. Now, after checking the otree hub log, I switched collectstatic back on and also tried deploying through github again and now it works. (I guess I had changed other stuff in my code in the meantime which must have affected collectstatic to not be a problem anymore...)
For future projects, can you tell why sometimes disabling collectstatic is recommended? While it might allow the build to be successful, doesnt that ultimately lead to some static filed not being able to call when running the app?