Hmm, the drops are indicative of pla thats been sat in the nozzle for a while, and go super hot and dripped (instead of extruded) out. As gr5 mentions, perhaps its the feeder and grinding. The UM2 has a direct drive feeder, so the stepper and knurled bolt sleeve get very hot, and can melt the filament slightly. This leads to excessive grinding and the appearance of a blocked nozzle, but is in fact the material not feeding correctly.
Its just a guess, I have a UMO!Edited by Guest
Did the print have lots of retractions? Sometimes that can cause grinding and a print fail. Or it could be a tiny bit of metalic or wood dust or house dust that traveled up the filament and into the nozzle and clogged it.
You might want to get an olsson block (google it) from my store as the nozzles are easy to unclog when it's a really bad clog.
I would remove the block and put it in a flame and burn the hell out of anything in there. If you buy anything from my store I always include a free nozzle cleaner tool with first purchase - I make the tool from a hypodermic needle that is .35mm diameter. Acupuncture needles also work well. When you heat the nozzle with flame be careful not to melt the brass even though you have a very wide margin between "burn things to ash" temperature and "melt brass" temperature don't let it get any hotter than necessary.
There are videos of how to take the head apart - the tricky part is often getting the temp sensor out without breaking it. I sell those also (as does fbrc8.com in usa). Some videos here:
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