quote from US forums
Okay so update for you all because I found this EU post about the same issue and they’re linking the cause to be unwrapping the 6 month gift mount on that character and that’s how that character became affected.
https://eu.forums.blizzard.com/en/wow/t/linking-mounts-just-says-something-unintelligible/485007
So I tested this out because my main character specifically hasn’t been able to link mounts for quite a few months now. I went to the trading post this month on a character that COULD link mounts just fine and collected the monthly reward which is a mount that needs to be unwrapped and what do you know, the character I did that on is also now unable to link mounts the same way my main character is. I’m adding this info here to keep this thread alive since the one I’m linking didn’t get traction and to add a confirmed cause of the problem.
This month’s trading post reward is a mount, which gets delivered into your mount journal all wrapped up in a gift box which you need to open. It adds up to the working theory and this is the most likely cause of the bug.
Two of my friends have successfully fallen victim to it / reproduced it but there may be an additional factor at play because the bug isn’t consistently affecting every player. One thing in common is that we all have looted a trading post CHEST in DORNOGAL (alternative ones are in ORG and SW but those could be faction specific).