SOLUTION HERE: I HAVE READ FROM SOMEONE BACK OFF THE PHONE WITH ROCKSTAR THAT THE PROBLEM JUST STARTED OCCURING, MESSAGES ARE POPPING UP EVERYWHERE. OBVIOUSLY THIS IS INTERNET RELATED, SO A SOLUTION HAS BEEN FOUND: DISCONNECT FROM ROCKSTAR SOCIAL CLUB. THIS WILL ENABLE SINGLE PLAYER UNTIL A FIX IS RELEASED FOR THE SOCIAL CLUB LOGIN.