Cannot close my own threads

Since recently, when I try to close a thread I created on an answer of mine, I get the following error:

Trying later, next day, does not solve it. In the list of all threads, the post is not marked as answered.

But if I look via Chrome or plain Safari, I see the answer itself is marked as correct:

Which is not the case when opening in Safari Technology Preview (Release 233) where the same answer is still to be accepted:

In fact, same issue when trying to accept someone else answer.

Cannot close my own threads
 
 
Q