Had this happen with the archon neck. A bit of WD40 didn't work and all I managed to do was cut my hand (I sympathise with you, that stuff is nasty)- we applied a cold pack to the coupler and a heat pack to the body of the saber. It let the two lots of metals shrink and expand enough that it unscrewed with a bit more convincing. Course with that we had the advantage of having something we could actually get a hold of.
Before you screw the coupler back on carefully clean both threads to remove any tiny filings or dust of aluminium or the anodising (if it has it) in case it is that jamming in the threads and as the others have said, use some lubricant to prevent it from happening again
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