You're unlikely to be able to identify the root cause that way. In simple terms, the AP is telling you the frame it received was unreadable. Probably the transmission was interrupted, or corrupt by background aspects, but the AP doesn't know why.
If your controller and APs were a lot newer, the spectrum analysis might help. But from a practical standpoint, I'd suggest you're a long way from that being an option!
So, you should look at the following.
1. How many clients are connected on that AP on average? Is it busy? If so, this might explain the drops due to contention.
2. Are all APs doing this, or just one? If it's all of them, it suggests configuration optimization is in order. If it's just a few, that suggest localized issues, maybe interference or noise.
3. What is the physical environment the AP is in? Near a lift, kitchen or industrial equipment? All of these might cause interference and drops.
Start from there, and consider physically visiting the AP and its surroundings if you're not sure.