Can anyone confirm this may be due to Progressive Backups being enabled on the server?
The issue was resolved by removing plugins that for some reason were not working on FileMaker Server due to incorrect permissions on the actual plugin file itself (fmsadmin and fmsserver users did not have full access to the files). The first thing you should try if you get this message is to remove all plugins from FileMaker Server and then add them one by one, while checking the plugin file permissions, to see which one is the offending item.
I am having the same problem today.
Mac OS X 10.8.5
Java 7 Update 45
It seemed to work just after updating the OS to 10.8.5 and updating Java just after that. But the script never ran after being set up, and it was removed from Schedules when I checked back.
Now I'm back to not being able to go past the point where the account is specified. I did quadruple-check the account/pw.