This is happening to my nephew, not me. He doesn't have a forum account so I'm asking for him.

Starting last Friday, his main character suddenly cannot mount his horse. The animation begins, you hear the whistle, the character turns sideways as if to get on the horse, then the horse does not appear and his character simply turns back around and is standing there.

We tried changing which mount he has selected.
We tried logging his account in on my computer, and the same thing occurs there, a completely different computer.
It happens on both Mac and PC.
He just made 40. We don't know if that's related but when we were shouting in zone a couple of other people who had recently moved into the 40s also said they had the same problem.
All of his other characters mount just fine. The same horse (the Mind-Shriven Horse purchased from the store, the Imperial Horse acquired from the Imperial Edition of the game, and the cheapest horse from the stables, Sorrel).
He's obviously logged out and logged back in. Multiple times. He's rebooted. But the fact that it happens on my computer, and also a Mac, tells me it's not a client problem.
He has tried zoning.
He has entered and exited combat.
He has dodge rolled.
He has hit ESC twice (which sometimes fixes things).
We have tried without any addons.

On my computer, I can mount on all my characters. If I log in that same client to his account, his main cannot mount, but all of his alts can. This is exactly the same symptoms he has on his own computer. The computers in question are not in the same house, though they are both connected via Comcast.

He opened an in-game ticket on Friday, but no response as of yet.