My latest finding:
I created a blank CB GroupJive group that has the same permissions and settings as the one in which the comments don't work.
If I upload an image in this group (root folder) and write a comment, it will also be displayed after a page reload.
But as soon as I create a gallery, upload a picture there and write a comment about it, all the comments are no longer visible after a page reload. Not even that of the image in the root folder of the group.
If I delete the gallery again, the comment on the first picture is visible again.
There needs to be some connection between showing comments and galleries