Well son of a gun, I never thought to disable sync on the main account.
Just disabled it and re-added my secondary, and sure enough it works perfectly. I guess it's time to try cleaning out my main account, though frankly I have no idea what it could be; I deleted/removed from my synced folder anything from the last week (the problem started Saturday).
I'll post again if I manage to get my main working again. Thanks!
Edit: I should note that I have cleaned out my spam folder earlier, as well.