For those who may be experiencing the same issue: I contacted customer services via live chat. The guy was really helpful, checked if I had done all the possible troubleshooting options (e.g. rebooting, factory reset). After syncing he saw the high heart rate results in the last few days and told me that it must be hardware problem. Luckily my watch is still under warranty so I was offered a replacement, which has been arranged instantly and already on the way to me. I call this outstanding customer service. The watch was 1,5 years old and I had extended warranty from the retailer but it was accepted by Fitbit to be eligible for a replacement. This saved me the lengthy procedure of sending it back to them.
