The option "Only receive private messages from users on my Buddy List." in thUser CP doesn't work when the buddy list is empty. It appears !empty($user['buddylist']) has been added in inc/datahandlers/pm.php for some reason but I don't know why.
The problem was with possibly empty $fromid but now that strpos() is used we don't need to check values, only make sure that $fromid is always an integer.
Hi,
Thank you for your report. We have pushed this issue to our Github repository for further analysis where you can track our commits and progress with fixing this bug. Discussions regarding this bug may also take place there too.
Follow this link to visit the issue on Github:
https://github.com/mybb/mybb/issues/2080
Thanks for contributing to MyBB!
Regards,
The MyBB Group