AOL server error 400 bad request is such an error that may happen due to errors in the server of AOL or corrupted Windows registry files. Now if you need to fix this you have to follow certain steps given below: • Check and try to repair windows registry entries associated with error 400. • Run the firewall and antivirus programs to remove viruses. • Access 'disk clean up tool' to delete junk files and remove temporary files. • Now update system drivers and review all the changes in windows. • Next, you should try to install all the available for AOL messenger app. • Also, you can try to repair missing or corrupted files with windows file checker. Further, you can get connected with AOL customer service and get the instant solution.