(This can be a result of mechanical impact, oxidation after prolonged use at high temperatures, or that the O-ring was out of position and was screwed into the thread.) Try one of the following:

a) Turn the cell upside-down if possible. Add a drop of fine oil to the thread and let it flow into the thread. Flow cold cooling water through the base unit if possible to make it contract. Submerge the outer flange in hot water or heat it otherwise to make it expand. Now try to unscrew using a cloth or wrench for grip.

b) Check the threads inside the flange. If they look dirty or contain outgrowths or particles, brush them with a stiff brush, tooth-brush, steel-brush, mini-drill steel-brush or similar. If a particular particle looks stuck, peel it out using a tweezers. You may add a droplet of oil also to get it going, but it should not be necessary once it runs. If you have a mechanical workshop I believe they will easily clean up the threads.