run file verification tool available at tools & maintenance section of admin panel to find missing
/ changed files. focus is on php & js files. ignore files reported from install folder & its sub-folders.
check the missing files at their locations. if files exist then check file permissions (at least CHMOD 644)
if files are missing then upload them from MyBB source files pack of your forum's version
Thank you, m. BTW, congrats on the promotion.
