Okay, I just solved it by running tcpdump on my Pi to figure out what was going on.
I have a special character in my password so I thought that might be the problem. It wasn’t, but:
My password was 9 characters long. The HTTP-GET request clearly was missing the last character.
After changing my password to 8 characters everything is now working fine again.
Is this a known issue with the mobile app?
Anyway, I can happily go to bed now knowing that OSPi is taking care of my garden again! 🙂