I recently went to the Athenium to fight the Loremaster but there was no team up option below the "Press X to enter" notification and remembered there was definitely a team up option. I thought it was a small bug and I re logged, and it didn't fix. I recently took a w101 break and maybe kingsile disabled team up for this certain fight while I was gone. But could someone please clarify as I'm only able to fight her by myself.
Is the option showing up for you on other sigils? I just went to Loremaster and the Team Up is working just fine for me. You might try reinstalling the game.
