It seems like I lost all my luck. Only a three weeks ago, the Yamaha TW-225 in Taipei had problems after my arrival. Now that I'm back to Berlin, my BMW F-650 had some serious trouble, too.
Starting the engine turned out to be really hard (started only on something like the 10th attempt, even though usually the first one is sufficient). Furthermore, pulling the gas handle only the tiniest little bit kills off the engine completely, independent of how far the choke is asserted.
So today I spent some five hours in disassembling almost the entire bike, removing the twin-carburetor, disassembling and cleaning it and putting the entire bike back together again. The engine is running fine again. I just wonder why I have this kind of carburetor problem already the second time in the last couple of years.
There's almost no visible dirt inside the carburetor, and all the fittings are fine, no signs of any leakage, no signs of any significant wear of any of the involved parts. Still, cleaning and re-assembling it clearly removes the problem.