You hand in a quest to an npc who then has a follow up quest to offer you, and sometimes (quite often these days) you will be unable to interact with them. In order to fix it you have to speak to another npc or do a /reload.
After handing in the initial quest the npc’s dialogue box disappears. But if you do a /reload you suddenly hear them say their goodbye dialogue. So what I guess is happening is that even though handing in the quest causes the dialogue box to disappear for the player, the system thinks you are still in that dialogue window with them.
This bug has existed for literal years now. I’ve seen countless others talk about it, both here on these forums and on other sites. It is appalling that it is still not fixed. I felt compelled to bring it up again now because it seems to be far more frequent since 11.1 went live. Sure, the workaround is simple. But it becomes a massive nuisance when you’re having to do it for every few quests.