Hi Leo,
thanks for your reply. We will check for any plugin conflicts and server caching etc this week. If the problem still exists, I’ll get back to you.
One thing I noticed which may be related: The owner of the project added some menu items (pages) via the customizer, which don’t yet exist. And he was very frustrated because the menu items didn’t update on the live site (as these pages are in customizer draft mode, so not yet published). So maybe there is a conflict related to that. I have never used the customizer to create not yet existing pages to set up a menu so far…
We are going to delete the whole menu and also check for plugin conflicts.
If the problem still exists I can give access to you (live site / temporary wp login).
Thanks, Sebastian