"0x80570015 (NS_ERROR_XPC_CI_RETURNED_FAILURE)"

Are there any calendars in your setup? If so, are they local or network calendars?

I've tried changing the TLS level,

Make sure to set it back to the original value.

I did change the TLS level back to the original level 3 and checked with my ISP (Cox) to confirm that they supported TLS level 3.....I have not installed any calendar applications other than what came with the standard level install of Thunderbird 78.8.0.....completely uninstalled Thunderbird and re-installed without result....I have 4 or 5 e-mail accounts including Cox, gmail and yahoo, they are not working either....it does show that TB is connecting to the servers, but is then timing out before e-mail is received or sent....

.it does show that TB is connecting to the servers, but is then timing out before e-mail is received or sent....

Do you have an anti virus? Suddenly stopped working with multiple providers post update points straight at security software and on windows that usually means a third party anti virus product, although I have seen similar issues caused by bank security software and the stuff the US military issue it is rare. Issues with anti virus product proliferate after every update. Just usually the vendor names change.

What anti virus are you using? I have heard recently of issues with McAfee and a couple of others including bit defender and perennially with Nortons..

I'm using AVG free

I went into AVG and added TB to allowed apps.....rebooted, but it did not change anything....still can not send and receive e-mail

Matt.....I went into AVG and turned off e-mail shield....and TB started working, so at least I understand the issue....thanks for pointing me in the right direction.....