If i remember correctly if you have Calamity, that one is causing the problems, but im not sure about this, i did twice a testing of the two back then some how.. i cant move when summoning few of Fargo and Calamity bosses but some few Clam and FSouls bosses doesnt have that glitch.

If you have boss checklist, disable it, for some reason that was causing the issue for me.

I don't think it's a Calamity problem, did a playthrough a month or two ago with both Calamity and Fargo's with a buddy and all the bosses spawned fine. But, I'm getting the same bug; the game freezes my character and doesn't respond to any inputs, though the squirrel is still able to move and attack fine, and modded menus work fine, too. Just can't move or close my inventory or anything, really. It also disables any unlimited buffs I have.