Hi, the difference between Jaikoz 5 and Jaikoz 6 on the Mac is that Jaikoz 5 uses Java 6 and Jaikoz 6 uses Java 7, and the change from Java 6 to Java 7 was quite a large one because Java 6 on OSX was under the control of Apple, whereas Java 7 is under the control of Oracle like it is under all other platforms.
However this is not the problem on Windows because Jaikoz 5 on Windows already used Java 7. I think more likely that this is a firewall issue, that since you installed a new version of Jaikoz it is using different binaries and these binaries are not being allowed to access the Internet whereas the old one were. Although you can access Musicbrainz this is via a browser, firewalls generally allow access to the internet through a browser but are less relaxed about access directly from other applications.
Please Temporarily disable firewall access to confirm this is the issue, if still fails try rebooting the pc with firewall off, if confirmed this is problem then you/me can work on configuring your firewall.
If this makes no difference please send your support files (Advanced:Create Support Files)