thank you so much! I talked this through another time with my institution’s IT support and we noticed that certain additional permissions make the server block the PHP entirely (in case somebody else with the same problem reads this thread: no writing permissions for group and global were allowed anywhere in public_html). Now it finally works!
I noticed that recordings are named after their respective mediarecorder element and saved together in a zip archive, which is perfect. Is it possible to automatically name the zip, too? By default, it seems to be a random order of letters and numbers, which would make it hard to connect the recordings with our meta data. Ideally, we would like to automatically name each zip after the TextInput where we let our participants type in their anonymization code (but if we had a possibility to include that in the file names or have an extra file in the archive that just contains the anonymization code, that would also do). I suspect it will have something to do with modifying the UploadRecordings environment, right?