So I don't have GREAT news for you but I do have a way to at least make your set up work for you. The BAD news is because of how WPF works there just isn't a great way to let the theme's know with a relative path WHERE their resources are. That is why we create the symlinks on startup, to get around that limitation. Unfortunately that work around doesn't function with exFat partitions. the GOOD news is I have a work around that SHOULD.
You will need to do the follow:
1. Make sure LaunchBox is CLOSED
2. Copy the LBThemes folder to \\LaunchBox\Core
3. Mark the \\LaunchBox\Core\LBThemes\ folder as readonly (when Windows asked if you want to push it down to all the files say yes).
Let me know if that resolves the errors you are seeing. It is important to note that if you do this, moving forward if you download a LB Theme you will need to play it in BOTH LBTheme folders, not just one.