in the meantime I took the defective device apart. To my big surprise it turned out to be a contacting encoder rather than an optical one as mentioned in one of Elecrafts emails.
Here’s the evidence:
Most likely its this one:
The operating range is specified from 70°C to -30°C. So it should not be the encoders causing the issue - except some of the viscous fluid visible in the first picture gets on the contacts and causes high resistance at low temperatures (?)
Anyway, I agree with Matt.
73, Roman - DL3TU