Synchronization error (0x800CCC0E) in Outlook 2016

As the title says, I am getting error 0x800CCC0E in Outlook 2016 whenever I attempt to check my email. The error started yesterday (10FEB2021) with no setting changes to Outlook. I am able to check email through the Mail app on my iPhone and through mail.yahoo.com.

I have confirmed that my mail account in Outlook 2016 matches the instructions at https://www.att.com/support/article/email-support/KM1086159. I am using the Secure Mail Key. I've also removed the account completely, restarted Outlook without it, shut down, and then recreated the account settings. Same issue.

If I use the Wizard in Outlook to set the account up, it ends with the error "Something went wrong" (real informative). However, if I go into Mail settings in Control Panel to set up the account, the process completes properly. I can confirm this by going into email on my iPhone or Yahoo and seeing the test message that is sent from Outlook.

I think I've narrowed down the root cause. I am running Bitdefender Internet Security. Under "Online Thread Protection", there is an option called "Encrypted Web Scan". When this is enabled, I am getting the error. Whenever I shut this option off, it works without issue. I am not sure if it's AT&T's server that is triggering this or something in one of the e-mails.

Thoughts?