I've had this happen at PQs as well.. It's when your reward is Lore and you're only allowed 1, and already have it on your person. Instead of showing you've won another "Dragonslayer +12" Sword, it just shows up blank like in your screenshot.
I'd, if I were the coder, would add a check for this state and if it checks 'true' there should be a second roll for another item; or at the very least, state a message that you would have received an item you already own and can't own two of.
actully it does show that , for example when i do the tradeskill daily i get a message that i already have thunderous recipe x and cant have more I have experienced this however with a reward saying i got a illegble scroll however nothing ever came in my bags.
If you have additional details of exactly what instance(s) this is happening in, and possibly any ideas about what the item was that it failed to give you, I'd urge you to to submit a support ticket and include those details. If it's happening on a specific mob, with a specific type of chest, include that info as well.