[Hall of Famer]

Come to think about it, you probably need to manually add a default .htaccess file that does not use URL modrewrite for your mybb forum's root directory to avoid this kind of problem. There's a .htaccess file in Mysidia's install directory, which does not redirect you to the root directory of Mysidia Adoptables. Upload this .htaccess file and you will not receive errors any longer.

Note: This applies to users who have installed MyBB inside their adoptables directory, if your MyBB forum is in a folder outside of the adoptables root directory, theres no need to worry about the .htaccess issue.