The bug that hit me and others in phase 4 was "unauthorized to summon mount". Basically I got the legacy chocobo whistle (moogle mail) and goobbue horn (calamity npc). I used both, which put them into the Actions and Traits window under "Mounts". When I tried to activate either action, from either the window or after putting it into the action bar, it gave that error.

I was able to use the Goobbue mount in phase 3, repeatedly, but the horn always stayed in inventory, which I thought was odd.

Anyway, hopefully they fix the bug or provide a workaround. I'll miss my goobbue very much (legacy chocobo, too) if it's permanently an action on my character that I'm never "authorized" to use.