I can't place items in stash when the slot I've clicked is occupied by an item of type Food and Drink, Ingredients or Quest Items.
Here's what I'm doing:
Step 1) Click an equipped item, removing it to my pointer.
Step 2) Click a stash slot.
One of three things can happen:
Outcome #1) Empty slot. The item goes into the stash.
Outcome #2) Mouse-over of slot produces white overlay. The item goes into the stash.
Outcome #3) Mouse-over of slot produces red overlay. The item goes back into the equipped slot. If I had placed a new item in the slot, the new item is returned to my pointer.
Also, when I click on a Food & Drink item, it generates the in-game error message box for, "Food can only be consumed when resting."
Fake Edit: I just loaded up the game and tried moving an item from personal inventory to stash and it produced the same behavior.
And I can't remove any of the problematic item types from the stash.
It looks like the problem is that, "Hello, I would like to place this item in stash," fails to supersede, "Sorry, you are clicking an irremovable item."