I also had this happen with my Scorpion EXO900. I had pulled into a rest area and when I tried to lift the chin bar, the release just pulled with no resistance. I tried everything to get that evil bucket off my head and it did not want to move. My riding buddy was laughing at me as I panicked more and more as claustrophobia set in. I finally got him to cooperate and help. I'm sure it was very funny to onlookers as I pressed my helmet to his belly while he took my chin straps and pulled outward as hard as he could. While he did that I was desperately pushing on the helmet. I thought I was going to rip my ears off (and those that know me know that my ears are pretty prominent!), but it finally came off. Scorpion responded very quickly with new parts and simple instructions to repair. But I have never been able to bring myself to put it back on even though I really liked it otherwise. That's been a few years ago, and it still sits in my closet. I finished the final hour or so of that ride with no helmet. Boy did I feel naked.
I had never paid attention before, but with the chin bar up, it is very easy to spread the lower part of the helmet and slip it off my head. With the chin bar down, it would not spread. I went back to HJC. While it is also a bit tight to remove with the chin bar down, it can be done.
I've never tried it, but I wonder (for future reference) if you took a pair of pliers and start yanking at your cheek pads, would they come out and give you enough clearance to remove the helmet easier?