I finally got it working after 7 hours and 40 minutes. After an hour, I at least got the tab to show up. But not the module.
I think I read everything 40 times and just couldn't find the error. Everything seemed to be as described.
Then I found the problem. I had the modules in my Joomla template position, and they were only visible on certain pages.
I then changed the setting to "All pages" and deleted the module position. And now everything works.
Thanks again!
