Hello,
I had the same problem with a Vokera Mynute 12e.
Problem: Vokera's Thermister came faulty from factory. I know for sure that this is happening for 12e models. Since the symptoms are the same for other models I assume the same might be happening to them.
Solution: Replace thermister, about 25 euros (£19) or even less.
No issues since.
I spent much more money with replacing the heat exchanger, the PCB, calls out from plumbers, flushing the system out, ... No success. In the end, the thermister replacement did the trick. Original (faulty) thermister is red (or blue, I can't remember now) and new one is blue (or viceversa). Call Vokera and explain the situation, they should know straight away and confirm the above. This part is responsible for switching off the ignition system should the boyler reaches a too high temperature. The faulty one does it whenever it "pleases it".
Hope that helps,
Luis