I don't have that tank, so not sure how much help I can be. Hopefully somebody who does will stop by. In the meantime.......
Did you remove the drip tip before trying to open it? If not, try that.
Did you have it open and fill it before?
Did you disassemble completely before using it?
About the best I got right now is remove the base, coil, and glass leaving just the top. Then run the top under hot water or maybe even soak it in some hot water for a bit. Dry it as best you can and try to open it before it cools clear down. I'm wondering if the fill gasket might have dislodged a bit and is putting it in a bind.