Just updated from 10.0.3 to 11.0.1, and all global-defined external storage folders become inaccessible by users.
For all of them we configured User and Group Permissions. Clearing the “available for” field, or inserting user names instead of a group names, solves the problem.
Thus it seems there is a bug in the latest version of “External storage support” app (v1.1.2), which prevent to assign external storage folder permission to groups.
The bug still persists in Nextcloud 11.0.2
I just submitted an issue in Github, hoping to speed up the fix.
Unfortunately I have not the development knowledge to address the problem by myself, but this issue is actually preventing us from upgrading to the 11.x branch. Anyone can suggest me anything about it?
Good news: I (blindly) digged into the code and I found the commit which broken that functionality! Now I need someone able to understand why it does that and how to fix it… thanks! 
Same issues here with some installations but removing “available for” field doesn’t help.
Any hint?