Sometimes, have been unable to understand why.. /backpack will attempt to open the inventory for a split second and then fail. 
This makes the plugin very cumbersome and frustrating to use for players.

There are 3 backpack types, but only small and large wooden boxes storage size are available. 1 and 3. Box size 2 does not change the box size.

hello, i dont use the different sizes but i think there are something wrong in the plugin with the permissions search line 83 and change from

(_instance.permission.UserHasPermission(ownerID.ToString(), "backpacks.use.medium") ? StorageSize.Small​

to

(_instance.permission.UserHasPermission(ownerID.ToString(), "backpacks.use.small") ? StorageSize.Small​

I'm sorry that i cannot be more helpful on the issue of backpacks opening and closing, I'm finding it very difficult to pinpoint why.

Will post again If I make any breakthroughs. Doesn't seem to be very consistent.

Thank you for the update!!