Glad you got it figured out.
The cable between the controller & pump is bi-directional and senses the speed of the impeller as well as other data regarding the pumps operations. The controller is smart.. The manual.. Not so smart. :lol: Then again, that's what we are here for and we also monitor & respond to support emails 7 days a week.
Interestingly, the F8 error has only come up a few times that I recall in the last year. Once with you and once on Facebook very recently. Both were resolved with a bit of explanation however the first one who encountered it didn't bother to check the manual to even see what the error was for. In his case, the silcone hose wasn't on the venturi port all the way. There was one other F8 error that I remember and again, it was an easy fix.
Unfortunately, the manual leaves a lot to be desired and I do agree there should be some better instructions in there for address potential issues including what to look for in the case of the F8 error. The conversion from German to English in manual makes it even more confusing. I need to get a soft editable copy of that manual so I can clean it up and add to it. There is a lot of info that I have put into the tuning thread here and the overflow thread that addresses some potential issues and make setup and fine tuning much easier for those who don't read this forum.
Either way, I am glad you got it straightened out. Having the pump down deeper does increase the head pressure on the pump and also creates a bit more resistance on the air draw which in your case exacerbated the issue for you.