Thanks you Christian for your message although a little late. A month
without heating was no option for me....Unfortunately I was able to get
in contact with Nest via telephone and they send me a new
Heatlink.Problem for now solved.
I also contacted Google helpdesk via phone yesterday and after quite a
long call with a very friendly guy (45 minutes) they told me they wil
send me a new Heatlink! My Nest is a little more then two year's old so
I am really happy with this solution!...