Had a leaking radiator and cooling reservoir, so changed both.
Filled and bleeded the cooling circuit. Now, the fan doesn't seem to go on. 10 minutes running stationary with some revving up, no fan. Riding around with lots of city traffic and a temperature above 35c, no fan. You can hear the water in the reservoir is boiling. There are no leakages of fluids, radiator cap is shut. Checked hosea, everything okay. One thing I thought about is a leak that brings down the cooling temperature to below 102c so the fan never kicks in.
Before, I never saw it boiling (nor payed attention to it) and the fan kicked in every once in a while.

The circuit to the fan is ok, tested in with a electrical bridge of the temperature sensor. Warning light went on and fan started.

As I'm nowhere near a Yamaha dealer, the last resort that will bring me home is to wire the fan to the battery for the next 15000km.

Any other suggestions what might be wrong?

If the above is the case then it could be the temperature sender, but much more likely is that you've got an air lock despite bleeding the system. How exactly did you bleed it? Have you re-checked the level in the radiator is up to the brim after having run the bike?

__________________