Hate to say this… but you should start by making double-sure every one of your drivers (audio, midi devices, etc) are appropriately selected and up-to-date, and, if they are, trash your prefs.
Save as much as you can, shortcuts and setup-wise as presets (they’ll be saved as separate xmls). The major pain there is that if you’ve changed your meter colors they’ll have to (in my case, anyway) have to be rebuilt.
Then go to your Home Account/Library/Preferences folder, and take “Defaults.xml” out. Copy it to the desktop. Restart and see if you still have the problem. If so, it’s not the prefs, and you can replace the one Nuendo will automatically regenerate with your old one.
I’m running DropBox with no issues. I’ve had some trouble with UAD-2 (I use a Satellite), but those issues had more to do with losing connection with the device; that’s largely (though not completely) stopped since I’ve gone back to 32-bit.
Housekeeping practices that have made a noticeable difference in cooperation between my UAD-2 stuff and Nuendo: running Onyx, and most notably, running DiskWarrior.
FWIW, I’m on Mountain Lion. I’m not saying you should update, but it is a difference between our systems…and there’s always the chance that one of your drivers is not fully happy with 10.7. So many variables, so little time… but these are the things I’d check first.
Good luck,
Chewy