Here is your storage usage:
Backups 0 MB
Emails 0 MB
Files 520.49 MB
Trash 479.49 MB
Logs 1.59 MB
PSQL 0 MB
MySQL 48.82 MB
Total 1050.39 MB
When you delete something through the Plesk file manager you may want to check the box to skip the trash. I cleared the trash for you, and you're under the limit again. You should be able to login now, but your websites may continue to show 503 errors for a while until the next Apache restart.