Can't import bookmarks

I installed Nextcloud 12 on NethServer7 with Samba4 account provider. After logging in I added a few applications. One of them is the bookmarks application.
In order to add my bookmarks in the nextcloud application, I exported my bookmarks from FF as HTML. I tried to import the HTML file and I see a comment that the import was succesful, but the bookmarks just show up.

How can I troubleshoot this?

There are a few reports on the bug tracker:

A first step would be a look into your logfiles if you get any error messages. Then check if it is the same like an already reported issue and add your information or create a new issue if there isn’t a matching topic.