If your active program window crashes suddenly or your system starts freezing for a few seconds at a time, it means the error 521 has occurred. If you want to fix this error, you will have to follow the troubleshooting steps mentioned below, and it will help you get rid of the problem
Steps you have to follow:
1) Check your PC driver
2) Remove the junk files
3) Complete Virus scan needs to be done
4) Check registry entries
5) Undo recent system changes
6) Reinstall AIM program
7) Update the window
8) Use Windows system file checker
9) Clean installation of Windows
These are the troubleshooting methods that you can use to fix AOL mail error 521. These methods are surely going to help you. But if these steps don’t come out to be helpful, you can contact AOL email customer service to get assistance from technical experts who will extensively guide you.

Solved:- HOW TO FIX AOL MAIL ERROR 521