Polaroid EE100: one stiff roller

My Polaroid EE100 has one of the two steel rollers getting quite stiff, and very hard to turn; the other roller seems quite free though. Yes, I have them very well cleaned, so no dry developer build-up there.

The roller assembly, made out of metal, seems to be clipped inside a red carriage, and I do not seem to be able to remove it from the carriage for further examination and cleaning. Any suggestion?