This shouldn’t help you at all, as you’ve said you’ve encountered the bug already and I have no idea how to reproduce it.
Short description: Sometimes, the game prevents you from adding or removing items to your backpack until you save and relaunch the game.
Reproductibility: I’m haven’t been able to reproduce the bug on my very few attempts.
System description: Played in browser, Safari 5.1.9 with various plug-ins and extensions running inside Mac OS X 10.6.8.
Detailed steps which led to bug:
I crafted a backpack.
I used the backpack without any issue for a while.
At some point, I opened the backpack Container window by double clicking the backpack icon in the Items window.
I added several item to the backpack by dragging and dropping their icon from the Items windows to the backpack Container window.
I attempted to add an additional item to the backpack by dragging-and-dropping it from the inventory window to the backpack contents window.
The in-game log displayed the message [on the top of my head] “An error was encountered”. The item was not moved to the backpack.
From that point on, trying to to add another item to the backpack using the previously described method resulted in the same message being displayed and the item not being moved.
Likewise when trying to remove an item from the backpack displayed by dragging-and-dropping or by right clicking an item icon in the Container window.
Expected result: The item icon is displaced from the Items window to the backpack Container window.
Workaround: As you suggested, saving and reloading the game solve the issue.
Particular circumstances: Just after the bug occurred, I glanced at my WiFi connexion icon: it so happens that I had lost my connexion to my WiFi network (and thus the Internet) when I did look at it. I can’t be sure when the connexion was lost: could be long before, right before or no more than a few minutes after. Reconnecting to the WiFi did not render the backpack usable again.