The most common cause of this is that the thread is totally bogged up with old coffee oils that have solidified into a gummy paste over the years, and the thing has never been moved from the one position so it is effectively "locked in" with that paste.
So yes, you may have to ....gently but firmly....force it. Your idea of a self tightening oil filter spanner positioned around the ring is good. But dont get nasty!
Just make absolutely sure there is nothing else in there anywhere that may be causing the problem before applying gentle / firm pressure.
Coffee is oily and there shouldnt be any corrosion, except if someone has poured water in there (dont laugh, weve seen it!) but then the corrision is usually restricted to the brinding burrs which will look quite rusty, not the alum / brass threads.
By the way I have given this advice with the best of intentions, relying only on your informatin, and without seeing the problem first hand. I wont take any responsibility if you should break something in your own efforts to rectify.
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