This is more of a code issue, and I’m not familiar enough with code project management softwares like Git to be messing with requests or whatever.
Anyway, it appears that Sponge’s SpongeDataRegistry attempts to perform a method call, CopyOnWriteArrayList.set using an Iterator as it’s parameter. This is invalid (as per the Java specification) and throws an UnsupportedOperationException, which causes the server to hang, and eventually crash.
Related: http://stackoverflow.com/questions/5612470/remove-elements-from-copyonwritearraylist
Javadoc 8: http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArrayList.html
Copy of the console error: pastebin.com/U07fXcQd
Copy of the Crash log: pastebin.com/LRZvqPY4
Perhaps I’m overlooking something? Let me know if I need to add anything else.