Yep, that did the trick for me. Sometimes your are just to blind to see. Thank you guys for your help. Phabricator works now perfectly.

Tried this Installation Guide, and it worked until the point where you have to do the Database upgrade. But it seems like the database user from Plesk (create for phabricator with full permission, thats what plesk told) has no permission to create new Databases.

@avivey followed an instruction from a plesk forum.
@chad not 100% sure if it counts as a shared host. I'm installing on a VPS with Root.

Downloaded via the git Repos. Added the Databases from ./bin/storage databases by hand in Plesk and perfomed the "./bin/storage upgrade --force".