Thank you for your reply.
I upgraded my forum from beta.13 to beta.15 a few weeks ago, and after the update I noticed an error message popping up when I tested the byobu extension for posting private discussions. But I was so busy with other extensions that I didn't focus too much on the byobu extension.
I also remember that when I used byobu to post private discussions on my forum in beta.13, the error message also popped up (the error content was different from that of beta.15), but after refreshing, the discussion was posted normally, only when I clicked "Post Private Discussion" the error message popped up.
The error I got on beta.15 was the one I replied to above. I tried to test posting a private discussion, I used the format "aaaaaa" for both title and content, but the error occurred after I clicked post.
I don't know how to fix this error now, because it started on beta.13 and continued on beta.15, so forgive me for not looking for a solution to this error back in beta.13.