This can usually be prevented by applying a thin film of petrolatum (Vaseline, for example) onto the thread before assembling.
When you grip a thin-cross-section ring such as a retaining ring or lens filter quite firmly, it temporarily flexes into an oval shape. At the two pinch points the diameter of the ring is reduced so that the ring pinches the OD of the lens thread, greatly increasing the friction between the male and female threads at these points. This is why gripping an overtightened thin threaded ring very tightly to unloosen it can make it impossible to unscrew.
Applying a tiny amount of penetrating oil to the assembled threads might help. If the OD of the ring is accessible, you might get an grip on the ring with a split compression wrench, such as sold by Japan Hobby tool. The trick is in finding one whose hole is only a few thousandths of an inch larger than the OD of the ring.
I had a similar problem when I bought a used DeVere 5108 enlarger in 2009. Someone had installed an early-model 135/5.6 EL Nikker into the DeVere threaded lens panel (39 mm Leica thread) which I wanted to remove. It behaved as though the lens had been epoxied in place. I tried seemingly everything.
I took one of my Japan Hobby Tool split compression wrenches and used it as a model to make a scaled-up version whose hole was about 0.002” larger than the OD of the chrome-plated solid-brass adapter. This is an integral part of the lens, but unscrews from the barrel on the old EL Nikkors of 135 mm and larger.
I placed the lens panel with the stuck adapter ring into a vice and used the homemade wrench. It broke loose easily without leaving any mark. A careful examination of the threads, both male and female, revealed no damage and no evidence of any adhesive. It had been overtightened which made the two threads lock together.
When two threaded metal parts are snugged together, there are microscopic “hills and valleys” on each of the two parts that can get jammed together, making it very hard to break free when disassembly is wanted. To prevent this, it’s the usual practice to always lubricate the threads with a film of grease before assembling. This makes a huge difference.
In the case of lens threads, petrolatum is a better choice. It isn’t actually grease. It’s described as parafinate, a type of wax, actually. I use this when assembling two filters or installing a slightly tight filter into the threads of a lens. This makes disassembly easier and helps prevent jams.
https://en.wikipedia.org/wiki/Petroleum_jelly
If you use a slot tool to turn the ring. Be sure to apply much greater force downward into the ring than the turning force. This is to prevent slippage, which could ruin the ring or much worse, the lens. This is the same principal as when using a screwdriver.
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