During gameplay, my backpack suddenly stopped opening. Unsure of what caused it, here is the error log (pastebin) from the server console every time I try opening it.

https://pastebin.com/Mdsr58AZ

Probably the compound on the stack is too small to contain all this data about spawners. I can't do much about this though.

Game version? any mods?

Game version 1.16.1, I realized this might be an issue in compression. My best guess would be:

I have the Apotheosis mod which lets me mine spawners using silk touch. I had around 10 or so spawners inside my backpack and spawners contain a lot of information (Type of mob, min/max spawn rate, player distance, max entities, spawn range, etc). It might be too much information to be compressed in the GUI and hence the size might have increased the cap.

I don't believe any other mod has anything to do with it since I only had some vanilla objects and apotheosis spawners in the backpack, so I will not bother listing all of them here. Thank you for the reply!

EDIT: Adding on to this reply, the backpack opens properly when on ground. It does not open using the assigned key when worn.

EDIT 2: I know the above since I died on purpose to get rid of the backpack but when I tried opening it when it was on ground, I didnt have any problem opening it. I removed the spawners and wore the backpack and was able to open it again with the assigned key without any issues.

Game version? any mods?

I have the same problem in 1.16.3, and I also have Apotheosis! And some testing tells me that yes, the problem seems to be caused by too much information saved in the backpack.