I would go with TruckDrivinMama's suggestion above if it's stuck because of bad threading or tightening it down too hard.
If it's stuck because of e-liquid which leaked and dried, I'd try working an alcohol-based hand sanitizer into the inside of the pan, and on the bottom outside of the ring, and then try to twist it off with a towel or wearing gloves. Hand sanitizer is what I use to loosen stuck adapters from cartos, and just generally as a cleaning agent for leaked juice on vaping gear.
Just generally, for problems like this, some sort of lubricant and a bit of patience go a long way. If you find you need to use something like pliers, be careful about too much pressure -- I suspect the ring would bend fairly easily.