Well, I solved it (or it solved itself while I tried things).
This is what I tried:
Removed apple id from iPhone (from the web)
Add the apple id to the iPhone (from the iPhone)
Uninstalled testflight
Restart the iPhone
Installed testfligh on my mac
You can see all apps you have tested (WHY IS THIS NOT SHOWN ON MOBILE NOR ON THE WEB??????????!)
Removed the apps I'm not currently testing from the mac testflight
Reinstalled Testflight on the iPhone
Open Testflight, it worked...