I am also incurring this error. Closing out and restarting the game does not fix the issue all the time, or even often. I will randomly get it to work once ever 10-15 restarts. This is making my game completely unplayable as I cannot change zone without the pet there.
Now after trying multiple times to get this to work from restarts I am prompted with a Party Management Window upon loading. 2 Blanks Characters appear, neither will go away.