Probably because of corrupted server files. Try completely uninstall MTA and it's server files then reinstall MTA and redownload the server files again. Had this bug several times, all of them resolved after re-installing or overwriting the server files with previously backed up server files.
Also another thing though it's very much unlikely you touched it or anything, make sure MTA debug setting is set on default.
Also another thing though it's very much unlikely you touched it or anything, make sure MTA debug setting is set on default.