A bug you say?
Right on the windshield! I'm ashamed to say we missed something massive in the 0.2 update of UMSC that may have been breaking peoples servers! Yikes!
That bug is the .JAR LINKS bug. If you've been using the .JAR Links form to get your .jar files and you just so happen to be trying to make a Spigot server, silly old us have accidentally linked you all to broken jar files! If you've been receiving the "Server stopped incorrectly" error, this is the cause of that!
How to fix?
We're working on update 0.21 so the fix will be rolled out with that, however in the mean time you can fix any servers you've created using spigot by getting the .jar off of the internet and not off UMSC.
Version 0.21 you say? What's coming in this update?
As of right now, here's the current changelog:
+ Added more supported routers for port forward (beta)
+ Added rollback option
+ Added support for Minecraft 1.8.8
+ Added new links to .JAR links
+ Added settings
+ Added backup settings
+ Added snapshot support (beta)
+ Added ads (can be switched off, only shown once per launch)
+ Updated Start Screen
+ Fixed Server Test memory leaks
+ Fixed bug where multiple forms could be opened
+ Fixed servers not using 64-bit java
+ Fixed Vanilla playerlist not working
Please let us know if you find any bugs by submitting a bug report, it really helps us!