Can you try deleting your Battle.net cache, and see if this helps? It might be a bad entry stored there. I've seen instances where a corrupted file caused issues for one game but not another.
Clearing your Battle.net Cache
1. Navigate to Mac HD/Users/Shared/Blizzard/
2. Move the Battle.net folder to the trash.
3. Empty the Trash.
4. Launch Diablo III
I was able to get around this by setting up a VPN server at home and connecting to it with the built-in VPN connection on Mac OS X.
If anyone would like a detailed tutorial, I can supply one.
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