The circuit board must get fairly warm over the years when the unit is working so more prone to joints drying out? That would fit with me swapping boards between units and having it work last year and then not the next.
Have you looked at what temp the webasto thinks it's seeing, mine is seeing a temp of 110deg. with the engine cold so it wont start, i haven't yet removed it to check the internal sensor.
I haven't yet, I'll probably try to replace the temp sensor first then move onto the webasto. Will hook up vcds and check the temp reading then. Many thanks