Can't start the engine online
 

In single player mode I have no propblem starting the engine.
mags on (both), fuel on, prop pitch set to fine, start.

Online, however, I sometimes have trouble starting it. Even if I try to use different throttle settings. The engine turns, splutters and stops.

Same goes for Spits, Hurricanes and 109s.

What am I doing wrong?

Sounds like you're doing it right. The only thing I can think of is your throttle isn't high enough. Try starting with 10-15% throttle.

and leave the magnetos alone, it is not needed to switch them on manually......

No it's a new bug with fuel Mixture. All the keys are mixed (old ones and the one you hve setted by yourself). When you switch to a blue planes (109 - 110) and this happens, make sure you press any keys that are related to the fuel mixture ctrl (for me from time to time it's even mixed with the ChatBAr windows :( )

For example in the 110, yesterday was the first one I saw both ctrl lever of the fuel mixt. pushed full bckward.

It also depends which mission or map you use. Some airfields may be bugged on stock maps/missions (e.g. northern central field on Steppe map).