Hello and welcome. Follow these steps please:
1) Download the latest version of the 7520 Handheld Operating system from the Nexttel BlackBerry support web site. Look here for it - https://www.blackberry.com/Downloads...B9D5EE16489088
2) Execute the program you just downloaded
3) Restart Desktop Manager - it should load now.
A "507" means no operating syste (or programs or data). I hope you had a backup of your data