the same for everyone

Yep super annoying.

Okay, so I figured out why the boss locks up - stay a while and listen...

When fighting elite characters they have the stamina bar - which indicates how much punishment the enemy can take before going into a "dazed" stun stance, where they have lowered armor and do not fight back, you guys know this. ;D
Heimlock has this too, only he has a demon transform in addition, a second bar which indicates how much time/energy he has in his "released" form, both time/his own attacks/your damage will decrease this bar by varying amounts before he transforms back into human.

The Lock happens if Heimlock is struck while in "dazed" stance while also being in his demon form, he essentially doesn't have a way to exit this dazed stun stance while demon, and while he is in his demon form - you cannot fully kill him. This is the lock.

The answer that many have come up with (which I also followed, thank you guys who tipped me off) is to wail at him while he is in human form, if he locks up while being human (it did happen a couple of times to me) it doesn't matter, cuz your damage decreases his health, not his demon form bar - so at the end you will move onto the next phase of the fight.
While fighting human Heimlock it doesn't matter if he goes into his dazed stance, but it does matter when he transforms - cuz the daze bar is lazy and sticks in the red for a moment after he transforms. If you hit him right after he transforms into the demon, he will by effect enter his dazed stance while in demon and will not exit it, therefore locking progress. This means all damage, from your spells, ticks from dots and damage from the arena traps - so keep him in the middle between two of the arena trap pylons so he doesn't get hit by mistake while transforming.
When he switches to demon, simply wait for a moment - the lazy red daze bar will return to green in a moment after he transforms, just kite him abit between the traps, BUT DON'T LET HIM GET DAMAGED!!
When the Bar turns green again, go ham. When he turns back into human, ham harder.
This is how I in one go broke a double digit fail run streak.

Hope this helps, Good luck! ;D