SK is getting banned now (It CAN run, Java just won't let it run without exceptions enabled) in up-to-date java 7u51 installs, where the user freshly installed spiral knights as well. People are starting to hit this issue on windows and mac now, just see your technical issues forum.
See https://www.java.com/en/download/help/java_blocked.xml
Specifically the reason why it doesn't run now:
Jar file missing Permission Attribute:
- Permissions Attribute verifies that the application requests the permission level that developer specified. If this attribute is not present, it might be possible for an attacker to exploit a user by re-deploying an application that is signed with original certificate and running the application at a different privilege level.
- Error code: Missing required Permissions manifest attribute in main jar
If you guys didn't update Java yet, make sure you don't go past Java 7u45 til this gets fixed on OOO's end.