I am not sure if this is caused by a mod interaction or not but for some reason on my Infinity Skyblock Server, players are unable to open their grave if too much time goes by or a server restart happens after they die. The graves themselves are intact in the json folder, however, the players are unable to open them.

Edit:
I have done some additional test with pure forge essentials and this does not happen.
I am going to test to see if I can replicate what mod causes this and find a work around.

Closing this as you must have fixed it yourself recently.

Not the case, I fixed a bug with graves spawning on 1.12 but this was a 1.7.10 bug. I will leave it closed as the fire has long gone cold on this issue and if this comes up again, a new issue should be created with relevant information.