Users can't use Flow Upload. Only Admins. Why?

Some users in my Nextcloud instance can’t seem to use the app Flow Upload to upload files/folders into our Nextcloud instance.

Admin accounts can use it and it works 100% for Admins.

But anyone logged in as a ‘User’, they see the toolbar icon, and they can click “New destination”. But in the folder selection dialogue which pops up, there are no folders visible, and no destinations selectable. And yes, there are plenty of folders there, and users have access rights to them.

I’ve tried every setting and adjustment I can think of, and read the material I could fins on this app’s github page. I can’t seem to get around this.

I do have a Nextcloud-wide setting, which prevents Users to share any files/folders to outside user. OPnly Admins can share. Might this have something to do with this?

Any ideas?
Thank you.