I tried different user (admin, user with all rights incl. admin rights), different themes. The last I tried is
- klick 'Home' button in the main frame
- klickt 'Index a-z'
- klicked an artist name
- klicked an album
-> got the songs listed
-> marked songs / also with unmarked songs
=> drop down below the list 'Selected songs...' - no 'Edit tags'

Thx, Tom
BTW: I also checked the access rights on the server - they're all fine!
Actually I run Subsonic on
- Deb8
- non-root user
- Nginx 1.9.10 as reverse proxy (SSL)
I don't see any errors in the logs nor do I get errors in the browser (DevTools console). I'm using Chrome / Safari on OS X 10.11.6 (cleared cache, cookies already).
PS: I'll post a screenshot of the screen where I tried (can't post images directly, have to put them to my own webspace and link them

)