@Gary nailed my problem at least. Turns out Avast Mail Shield does not like emClient v8 for some reason. Not sure why yet, but disabling Avast Mail Shield allowed emClient v8 to connect to IMAP accounts and pull in mail. Once the initial connection is done, enabling Mail Shield appears to work again. Never experienced this problem with previous versions of emClient. I’ve had the same mail accounts for a long time, and used emClient with Avast since v6 as well.
The key difference here appears to be that emClient v8 manages certificates in a different way than v7 and prior. Avast automatically adds its root certificate authority to the Windows system certificate store, but Avast v8 no longer appears to use that cert store. I had to manually add the Avast Mail Shield certificate to emClient’s (separate) certificate store and then the mail shield worked.