Bug report #40788 which is currently marked UNCONFIRMED.
The error thrown is "Access Denied." I haven't the foggiest idea where to start looking to try and fix this. My other external accounts are fine (like GMail, generic IMAP, etc.) Only Yahoo has this issue.
I know that Yahoo requires something additional sent which was obviously included in ZCS and Zimbra Desktop. But I don't know enough about that to dig any deeper.
I recall having some issues with my upgrade to 5.0.18, but I don't remember what those were. It took some work to get everything back up and running. So it seems that the Yahoo IMAP access was a casualty of that. And that subsequent upgrades have kept that "setting," whatever that was. But there were a couple of others having the same issue. Hopefully they will chime in.
What file(s) could have gotten whacked?