Here you go, sbbs.el person, since you opened access to the tickets.
https://fossil.textboard.org/sbbs/tktview?name=ed8a04be6b
http://textboard.org/mona/12
Args out of range: "https://www.youtube.com/watch?v=ZLr8ntnL__A", 2215, 2217
The error was there before the UTF-8 fix, it just didn't crash because previous match bounds were small. The error is that in sbbs--insert-link -> let* -> other you are asking for match-string unconditionally, even when there was no match. You guard against this above the let* and in 'func', but you need to do so in 'other' as well. Evidently, you should never try to extract match data from inside a non-match.
https://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Match-Data.html
Someone might be nice enough to notify the sbbs.el person.