BuddyPress.org

The first thing to do would be to deactivate all other plugins apart from BuddyPress and see if the fault remains. If it does then you need to check if you have overloaded the file by looking in wp-content/themes/your-child-theme/buddypress/members/single/messages/compose.php, if this file exists then check permissions and file content, if it does not exist then check permissions of wp-content/plugins/buddypress/bp-templates/bp-legacy or bp-nouveau/buddypress/members/single/messages/compose.php