I'm not sure how to report this, other than that I got a message from the NPC in the post title trying to take the relevant quest. It went, more or less as follows:
"Nothing! Don't worry! // OOC This should never happen, please report this as a bug."
I tried again and took the quest no problem. There was no ding noise, weirdly. But no problem otherwise.
This has also happened to me. After seeing the "This should never happen" option and exiting conversation, a subsequent conversation worked correctly.