Bosch: Error 01 looking for VR=2.5

[OTGvis]

I'm using the Bosch TF30 and HRC 30 with OTG. Unfortunately the OTG gives a lot of Error 01 @ VR=3 already whith the initial testing (http://otgw.tclcode.com/debugging.html#slave). Setting the value VR=2 fixed the problem however after a while the communication was stopped completely.
So perhaps the best setting is in between 2 and 3 (just guesssing). Is there a way to have the voltage changes more granular, so VR=2.5 or something?

[OTGvis]

After 3 weeks running (can see the boiler temp, flame status, DHW, not able to set anything yet - using VeraLite) I installed the Diagnostic firmware. Used the bit test #2 where the thermostat quite often fails with the only last byte < 900 (but close), the boiler interface #3 is al lot of red, lots of 750 etc.

Does that mean a nog-go for setting anything with the Bosch HRC?