Hey there,
just finished a dungeon with one person offline, tried to leave group, but couldn't. The symbol under the character portrait wasn't clickable and I noticed that anything else also wasn't clickable after I tried to click it until I opened another in-game window (e.g. Inventory) again via keyboard shortcut. (Hovering over inventory items did not show their descriptions was how i noticed it) Even hitting the escape key multiple times did not solve the problem.
The other two players could leave but I was still stuck with that one offline person and couldn't continue to group up for more missions.
Tried to relog through character selection, still the problem.
Tried to relog through server selection, still the problem.
Tried to relog by restarting the game, fixed it.
^In all three cases I was still in the group with the offline person.
Even though I could leave the group in the end, I think I should notify that there can come up a problem with leaving groups after a mission.
Greetings,
Cryo