Update: After changing the STORAGE_ENDPOINT_URL to http://minio:9000, I tried again to create a new project. It now works and can be synchronized with the cloud in QGIS. However, I cannot synchronize it in Qfield. I now get a download error there (no further information are provided). The docker compose app logs now show a new error:
The already existing projects are still not syncable either in QGIS nor Qfield.