problem(s) solved:
Just for Reference. There were some changes that i might have not seen in the changelogs:
1st: i put all seafile related datablocks on an external storage / nas. The Nas OS does not allow to chown the directories that are stored there. thus i have to change my local debian user to fit the nas user/group ID. Changes in the seafile server seem to prevent that the user running seafile is not the same than the user owning the data directory. i just switched the startup script so that seahub and seafile did run as seafile user (which was root beforehand - better practice anyway)
2nd. after downloading the new release (as root user) i forgot to chown the seafile-server-6.3.4 folder to seafile:seafile so that (1st) could be applied.
3rd. i had to downgrade urllib3 from 1.24 to 1.23 for the script to run.