Another one:
Dungeon: Ascalon Catacombs, story mode.
Problem: Can not rejoin the instance that the rest of the party is in after a disconnect. A window appears, asking me to “join my group”, but when I click on the “Join”-button, the window vanishes and nothing happens.
When walking into the instance portal, the window appears again. In this case, clicking on “join my group” will land me in a new instance of the dungeon, without the party.
Error persisted after logging in again, changing character, rejoining area, rejoining party, etc. Error could be reproduced infinitely.