Good news, Robin.
I just tried again, and again the “invalid credentials” result popped up. I tapped “OK”, and right away tapped “Login” again, now it started looking for devices and I could add the inverter.
Diagnostics: 8ef9a43d-d693-4023-b49a-18aa8cc4a4e3, maybe it reveals something useful.
I built a check flow now:
