My fault, following your advice I notied SeafDav is working just fine. I somehow mixed the problem with fuse problems I have and during my investigation for "Unknown erorr" and "Server error" status syncing libraries since Seafile Server CE 7.0.5? I found this topic.
However: I still have the log error Failed to init seahub db: No module named seahub_settings. but maybe this is more related to Seafile 7.0.2, seafdav bug "No module named seahub_settings." - #5 by shady2k.