When the Backpacks are opened, the inventory behaviour gets glitchy, after several moved items by D&D soemtimes the item gets stuck at the cursor and can only be dropped and not beplaced back into any slot

I think I've found somewhat reliable case to reproduce the issue.
Try to pick and place the same item over and over again in different slots. Not shift-click, but exactly pick-move-put-pick-move-put.

When backpack is open, shuffling your inventory around is a problem. Close backpack and you can move items around without an issue.

It happens intermittently, but items aren't "stuck", you have to click multiple times to get the item off the cursor. I only encountered it in MP so far.

I will rewrite container code asap.

Fixed via aa190f3

Is there a reliable way to reproduce this?

Cant reproduce. Seems fine to me