Happened to me too. In, addition the first time that I used my ult, the targeted for throwing the fire didn't come up and I wasn't able to lob any fire.
Match ID: 1029532031
It occurred while I was trying to clear the first red buff so sometime around 1:00.
I had it on normal casting.
No special key binds, just 2 in order to use the potion.
I had a health potion in my consumable slot, but when I tried drinking them and using the free space to store the potion, it let me store the potion, but I was still unable to use it. This happened at 4:30. Furthermore, I tried to brew a new potion, and that one didn't work either. That happened at around 4:55.
I tried killing myself to tower, but that didn't work. The only thing that worked was leaving the game and coming back. Thank god for SSD's.
Gonna copy/paste this to a lot of people - unfortunately Reddit isn't great for individual responses.
As an update we shipped a fix for this to Xbox/PC/PS4. It was a client fix, and server didn't need a restart, so you'll have to relaunch your clients to get the patch (was sent out at ~1pm EST) . Let me know if you run into this issue again after you've downloaded that patch!