I too had the exact same problem a few days ago on a client's PC. Kaspersky Internet Security was the culprit. I simply turned off the Mail antivirus protection module and the problem went away.

Thanks Stans, but,,, TBird says,, 'checking mail servers capabilities', then I get the error. I stopped Malwarebytes and Spybot and CCleaner incase it was doing the same, but no, my problem still exists:-(

Thanks Stans, but,,, TBird says,, 'checking mail servers capabilities', then I get the error. I stopped Malwarebytes and Spybot and CCleaner incase it was doing the same, but no, my problem still exists:-(

That's exactly what was happening on my end as well. Accessing any folder other than Inbox resulted in the exact same error, for a Yahoo account only. Are you running any other AV alongside Malwarebytes, say, Windows Defender? Sometimes simply turning off the AV doesn't cut it because its drivers are still loaded even when the shields are down. If security software is responsible as was the case for me, you can confirm by rebooting Windows into safe mode with networking. In safe mode, none of the AV components are loaded.

Yes Stan, You've hit the problem - AV is the cause. Email is allowed through the Windoze firewall, but the AV apps turn themselves back on! If I quickly turn them off, W Defender too, then all is OK, but seconds later they're back on and the error re-appears - Doh!