$thumbnail = generate_thumbnail(MYBB_ROOT."uploads/".$attachment['attachname'], MYBB_ROOT."uploads/", $thumbname, $mybb->settings['attachthumbh'], $mybb->settings['attachthumbw']);
As you can see that code expects the attachment to be in the default directory. However we have a setting for that ($mybb->settings['uploadspath'])
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/2104
Thanks for contributing to MyBB!
Regards,
The MyBB Group