While playing on a server, and after I died and respawned, the backpack was sitting on the ground, and when I right-clicked it, it was fine, however; when I left clicked it, the game would crash. I tried to return to the game and retry however the same thing continued to occur. I attached a video of what would happen when I tried to left click.

Thanks!

Can you send crash log?

I have a similar issue, but it happens when I try to place the backpack on the ground. Here is the log:

crash-2020-12-29_17.22.17-client.txt

Seems to be a compatibility issue with the Fairy Lights mod - in my case at least. I disabled Fairy Lights and was able to place and pick up the backpack without crashing.

Here's the crash log:
crash-2020-12-31_01.15.01-client.txt

Update to 5.3.3