People have been having issues with the ladder hatches, garage doors, and chests among other items becoming unusable after being placed in a backpack. They can equip it and put it in the hotbar but it will not be able to be placed.

also having this same issue

Did you delete planners on your server or run a cleaning program? I had this issue (not just with backpacks) when I went over the entity limit or ran a del of planners or ran a buggy cleaner program. Otherwise I have had zero issues with Backpacks with this. I suspect there are some other factors in play causing this malfunction. Have you done any of these activities?

I am using the Broken Items Cleaner plugin from Orange and have been for quite some time... If this is what's doing it then I will 100% get rid of it. Now that I think about it, I don't recall having this issue prior to getting that plugin.