I can’t, as an admin, change ownership of a topic. And safe-mode didn’t bring any clearence. I’m on iPad, so can’t tell if there is any errors in console. Diacourse is upgraded about an hour ago. What else… in that category is no unusually limitations, visible for everyone and writing is for TL1 — but I tried change ownership to staff and ordinary TL1 without success.
I’ve just had a test run and it worked for me on my test site. I could successfully change an OP to a different user.
By any chance, do you have the ActivityPub plugin active on that topic’s category?
Oh damn 
Of course. And yes I have.
처음에 Contribute > Feature 카테고리에서 이 문제를 마주쳤을 때 꽤 당황했었습니다. 
ActivityPub 플러그인의 현재 한계(혹은 특징?)이지만, 무엇이 일어나고 있는지 파악하기 전까지는 정말 헷갈릴 수 있습니다.
I kind of understand it with the very first post, because it is delivered. But I can’t change ownership of any posts under such topic and that bothers me a bit because ActivityPub should not see those - at the moment anyway.
(Let’ connect topics together: ActivityPub Plugin )