This was a reason to take the whole extruder apart to check what was going on inside. When taking the top plate of the extruder and removing the motor, the ball bearing came off... with part of the pulley.
The pulley should look like this (photo from makerbot flickr pool).
The thin flange of the pulley came off and after removing the small shreds of aluminium the flange is running loose around the rest of the pulley. I did put the extruder back together with the loose flange, but the bearing was pushed away again after extruding less than 5 cm and the filament stopped moving again.
I have seen many broken extruders in the blogs, but usually they where related to the insulator leaking, the retainer plate breaking, or the lack of grip on the filament because of the idler wheel slipping away. I did find some references to the same failure (e.g. here), but the best indication that this may not be such an uncommon fault is the fact that Makerbot is selling the pulleys also specifically "if you have broken your pulley"...
It is actually not so surprising that this part breaks. The idler wheel is pushing hardest in the middle of the pulley, so the filament will try to take either inside corner of the pulley to get away from it. This will create a serious sideways force. The flange of the aluminium pulley is only press-fit on the pulley and it looks like it is not strong enough. I guess the solution is to get the new MK5 drive gear upgrade kit which is made of one single piece of stainless steel and it looks much stronger. Possibly I make something similar myself.
I had already started work on a new "Wade" type extruder, but this is not ready yet. I will try to finnish that asap to get a functioning printer again, but I do want to fix my MK4 too, since having a spare extruder will come in handy someday.
Some parts printed when the pulley was still ok: the Hemi-Demi sphere from thingiverse. These parts:
will make this sphere:
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