Buried Shrines - Cannot open guarded sarcohpagus

Hmmm... yeah, bottom up or top down entry into room, un-clickable. Come at it from left or right, viola! (or some other stringed instrument), clickable! ...and I got stuck in the pack. HC SSF, ah well, yet another new character in-coming : 0(

I had the same problem. But, when I walked around behind it, and clicked it repeatedly, it did eventually open.

Can confirm Multiple clicking worked for me. After like 15 sec sarcofagus opened and mobs activated.
edit:
On other character multiple clicking didn't work but what worked was click->move slightly->click again. Two steps later i was able to activate it. It look like u need to stand in a right spot to activate it.