NextCloud migration to docker on QNAP NAS

I have migrated the NextCloud application from the package that is installed in the QTS itself (downloaded from the club repo) to NextCloud in the docker. It worked and the base is kept at native NAS MariaDB all the time.
On the NAS itself (and not in the docker) there are also files of documents, photos etc. Everything seems to be synchronized with, for example, the NextCloud Linux client. Similarly, access from the Linux filemanager to the files in the database works (via WEBDav).
However, for clients, access to the database is as if in read only mode. It is 100% a problem with some access permissions.

Can my colleagues tell me what to check?

I saw that you got this resolved on LinuxQuestions. Do you have a guide that could facilitate this process from start to finish? Or any recommendations?