Not sure if this is backpack plugin or sometihng else.   Some (not all) players hit to open the bakcpack and it just flashes open and closed in a second.  Those with the problem also can't open containers of any sort, or the /bank plugin for VIP.  It's affecting three things.   Not a clue what's going on here?

The Backpacks plugin itself cannot cause this. If you suspect that Backpacks (or another plugin) is contributing to the issue, you can try unloading the plugin to see if it resolves the issue, to narrow down potential causes. If you find that the issue doesn't happen while Backpacks is unloaded, it's probably not Backpacks itself causing it, but rather an interaction with another plugin.

It's worth mentioning that nivex (another plugin developer) recently reached out to me about a similar-sounding issue some users were seeing while using both Raidable Bases and Backpacks, where as soon as a player (who had gather mode enabled in their Backpack) tried to move an item from a container to their inventory, the loot panel closed. That issue turned out to be caused by some incorrect handling of the CanOpenBackpack hook in the Raidable Bases plugin. I advised nivex on the solution, but checking the latest version of Raidable Bases right now, it seems the fix hasn't been released yet. You can reach out to nivex to see if they can release their new version or share with you a minimally patch.