That fixed it Thank you!
I've a laptop and a PC, both Windows 8.1 but one is installed with german language and the other with english. Both are in the same time zone (Amsterdam, Berlin...) but the language difference makes the time look different e.g. 8:30PM vs 20:30. When I try to connect both systems via Bittorrent Sync, I get an error. See this image for the error and also the time (which is the same on both systems):