The “nonzero status 75” means that rt-mailgate (from your system aliases file) isn’t communicating with RT properly. Check the sendmail logs to find out what it is. You have the complete timestamp so this should be fairly easy to find.
If you can’t find anything definitive try adding --debug to the end of the alias and regenerate the file.
Around here 9 times out of 10 it’s a typo in the URL or the credentials.