Sadly, I don't have any clue what caused it or any hint of reproduction steps. It seems to be caused by me trying to ult Trundle while standing in his slow, but for a while, I was completely incapable of using skills for most of the game. I was able to autoattack and move, and I could cast my E, but as long as it was active, I could not autoattack (so I could not technically use it), nor was I even able to recall. What ultimately fixed it seemed to be Flash. Closing and reopening the client did nothing.
Here's the two r3dlogs. (Two since I tried to fix the issue by restarting my client)
[Game][Major]Vi unable to use skills
Here's the two r3dlogs. (Two since I tried to fix the issue by restarting my client)