If you rotate the collar only do it with the hot end at 180C because you can break it without all that much force and PLA is probably in the threads right now. However unless you take the whole head apart (probably only need to remove the fans) then I wouldn't touch it.
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illuminarti 18
it's probably just plastic curling back up under the fan shroud when the print starts to go wrong. It's not super-common, but it has happened. See this other recent thread for info about getting it out:
http://umforum.ultimaker.com/index.php?/topic/5710-um2-extrusion-fail/
When you put it all back together, you might try carefully turning the metal 'collar' that has the holes in the side, so as to move the nozzle down just slightly further. There should be a gap of 1mm between the bottom of the white teflon part, and the top of the 'collar'. Be sure that you don't set it too low, though, and push the nozzle block up against the bottom of the fan shroud.
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