Tried alt+f4 and relaunching. Tried logging into another character and got the same. Abandoning quest and retaking, disabling all add ons, ect. Nothing works, and I can’t complete the dungeon without being able to mount.
Need more info, where are you when you try to mount? Might seem like a dumb question but I’ve seen people get mad because they try to mount at the entrance and start raging about the place being bugged when it doesn’t work.
Also have you tried switching flight styles? Both types of flight work and switching might kickstart the ability.
As for the error, that’s a noodle scratcher. Maybe bug report forum or GM ticket, but honestly not sure either of those would get you a fix, at least not on time, if at all
There is another option to choose a dungeon as a weekly, that’s part of Call of the Worldsoul (which isn’t Dawnbreaker this week), this says it must be done on H+.
The one offered by Biergoth, I always do as a follower dungeon, have not done it this week yet.