first of all, this is a great api. We use it for about half a year and it is very handy :-) great work!
Now we get an Exception in a multithreaded environment. I analysed it, and found out, that it occurs when the system properties are modified, while a owner Config object is created:
Caused by: java.util.ConcurrentModificationException
I wrote a junit testcase to reproduce it. And I also found a Workaround for it, by use another SystemProvider (from your api) which take a copy from the system properties made with an enumeration, because enumerations don't throw ConcurrentModificationExceptions.