Drop it in a cup of hot water. [drinkable hot coffee temperature] for an hour.
Really? Black, or with cream and sugar?
Sorry, I couldn't resist. I wouldn't do that. But I would suggest that the grip knobs need to be tightened just to where it stops, not tight.
What I would assume has happened is that it was tightened too tight after being filled and the grip knob is stuck on the brass thread bushing, so the bushing unscrewed as the knob was turned back. Warming gently over a hair dryer (warming, not heating) and then gripping the brass and the knob with a grippy pad and turning CCW should loosen it. The challenge though will be inserting the piston mechanism into the barrel and getting it snug again without damaging it if you don't have a spanner to fit in the notches. The piston seal should be lubricated again before you put it back in the pen.
If you aren't mechanically inclined, MB service, or a good pen mechanic would be a wise choice. This resin can be brittle, and can not be solvent welded.
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