Sorry I am a bit late on this Leif. If the end of the knob has come unscrewed and the replacement is of the same thread then I would suggest a thorough degrease of both threads and use Loctite. 270 is for permanent jobs and 242 is for removeable jobs.
I am guessing that although this was stated as being captive and in view of the replacement knob that this was designed to be loctited in assembly. Either an out of date Loctite or insufficient cleaning prior to assembly caused the joint to fail. With fresh 270 applied to clean threads you would have to apply a lot of heat ( blowtorch job ) to break the resulting joint. Loctite like all these type of adhesives has a shelf life and should be kept refridgerated in storage.
Don
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