See if info here helps: https://support.mozilla.org/en-US/questions/1091068

Is that an error in Thunderbird or Firefox? Firefox error that sounds like yours: https://support.mozilla.org/en-US/questions/1214366

Dear Toad-Hall, thank you very much but your advice did not help. This is a Thunderbird problem. I tried to find any pieces of advice entering error codes but didn't find anything useful. I also tried to apply some recipes that solve similar Firefox issues to Thunderbird but was unsuccessful.

Do you use BitDefender? BitDefender's Protection Shield. If yes, switch it off and retest.

I use Kaspersky Total Security. I have tried disabling it, I have tried Safe Mode booting with network drivers, none could help.

What version of Thunderbird are you using? Is there anything cert related in the error console (Ctrl-Shift-J)?

Thunderbird 24.1.1 User Agent Mozilla/5.0 (Windows NT 6.2; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.1.1 Build ID 20131118083805 Console contains some crazy stuff: Errors: Please do not load stuff in the multimessage browser directly, use the SummaryFrameManager instead. resource://gre/modules/summaryFrameManager.js Messages: Could not read chrome manifest 'file:///F:/Program5&20Files*20(x86)/Mozilla%20Thunderbird/chrome.manifest'. Could not read chrome manifest 'file:///F:/Program%20Files*20(x86)/Mozilla%20Thunderbird/extensions/%7B972ce4c6-7ee8-4474-a285-3208198ce6fd*7D /chrome.manifest'. While creating services from category 'profile-after-change', could not create service for entry 'Disk Space Watcher Service', contract ID '@mozilla.org/toolkit/disk-space-watcher;!' While creating services from category 'profile-after-change', could not create service for entry 'Disk Space Watcher Service', contract ID '@mozilla.org/toolkit/disk-space-watcher;1' While connecting to services.addons.thunderbird.net:443 an error occured. Secure connection with this node failed: no common cypher alhorythms. (Error code: ssl_error_no_cypher_overlap) ((this one was in Russian language. I translated it myself. )) Security violation attempt: content «about:neterror?e=nssFailure2&u=https%3A//services.addons.thunderbird.net/ru/thunderbird/discovery/pane/24.1.1/WINNT%23%7B%2522%7B3b4e7bc6-3b45-11dc-8314-0800200c9a66%7D%2522%3A%7B%2522name%2522%3A%2522Aquabird%2520Black%2522%2C%2522version%2522%3A%25221.0.5%2522%2C%2522type%2522%3A%2522theme%2522%2C%2522userDisabled%2522%3Atrue%2C%2522isCompatible%2522%3Afalse%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7B972ce4c6-7e08-4474-a285-3208198ce6fd%7D%2522%3A%7B%2522name%2522%3A%2522%25D0%25A1%25D1%2582%25D0%25B0%25D0%25BD%25D0%25B4%25D0%25B0%25D1%2580%25D1%2582%25D0%25BD%25D0%25B0%25D1%258F%2522%2C%2522version%2522%3A%252224.1.1%2522%2C%2522type%2522%3A%2522theme%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7B2932f7e2-2676-0311-1416-09aa55951a97%7D%2522%3A%7B%2522name%2522%3A%2522Adobe%2520Acrobat%2522%2C%2522version%2522%3A%252220.6.20034.39303%2522%2C%2522type%2522%3A%2522plugin%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7Bb4a46c11-2945-5c38-9640-940aeb02e416%7D%2522%3A%7B%2522name%2522%3A%2522Intel%25C2%25AE%2520Identity%2520Protection%2520Technology%2522%2C%2522version%2522%3A%25222.1.42.0%2522%2C%2522type%2522%3A%2522plugin%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7B37ab856c-b575-a4b5-33b2-9adc43f4ad10%7D%2522%3A%7B%2522name%2522%3A%2522Intel%25C2%25AE%2520Identity%2520Protection%2520Technology%2522%2C%2522version%2522%3A%25222.1.42.0%2522%2C%2522type%2522%3A%2522plugin%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7Bf23a76ab-1626-d214-3458-df56a2c9e828%7D%2522%3A%7B%2522name%2522%3A%2522Microsoft%2520Office%25202010%2522%2C%2522version%2522%3A%252214.0.4761.1000%2522%2C%2522type%2522%3A%2522plugin%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%2C%2522%7B85ad01e6-c414-e805-729c-ac9c1238914d%7D%2522%3A%7B%2522name%2522%3A%2522Microsoft%2520Office%25202010%2522%2C%2522version%2522%3A%252214.0.4730.1010%2522%2C%2522type%2522%3A%2522plugin%2522%2C%2522userDisabled%2522%3Afalse%2C%2522isCompatible%2522%3Atrue%2C%2522isBlocklisted%2522%3Afalse%7D%7D&c=UTF-8&d=%D0%9F%D1%80%D0%B8%20%D1%81%D0%BE%D0%B5%D0%B4%D0%B8%D0%BD%D0%B5%D0%BD%D0%B8%D0%B8%20%D1%81%20services.addons.thunderbird.net%20%D0%BF%D1%80%D0%BE%D0%B8%D0%B7%D0%BE%D1%88%D0%BB%D0%B0%20%D0%BE%D1%88%D0%B8%D0%B1%D0%BA%D0%B0.%0A%0A%D0%A3%D1%81%D1%82%D0%B0%D0%BD%D0%BE%D0%B2%D0%BA%D0%B0%20%D0%B7%D0%B0%D1%89%D0%B8%D1%89%D1%91%D0%BD%D0%BD%D0%BE%D0%B3%D0%BE%20%D1%81%D0%BE%D0%B5%D0%B4%D0%B8%D0%BD%D0%B5%D0%BD%D0%B8%D1%8F%20%D1%81%20%D1%8D%D1%82%D0%B8%D0%BC%20%D1%83%D0%B7%D0%BB%D0%BE%D0%BC%20%D0%BD%D0%B5%20%D1%83%D0%B4%D0%B0%D0%BB%D0%B0%D1%81%D1%8C%3A%20%D0%BE%D1%82%D1%81%D1%83%D1%82%D1%81%D1%82%D0%B2%D1%83%D1%8E%D1%82%20%D0%BE%D0%B1%D1%89%D0%B8%D0%B5%20%D0%B0%D0%BB%D0%B3%D0%BE%D1%80%D0%B8%D1%82%D0%BC%28%D1%8B%29%20%D1%88%D0%B8%D1%84%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D1%8F.%0A%0A%28%D0%9A%D0%BE%D0%B4%20%D0%BE%D1%88%D0%B8%D0%B1%D0%BA%D0%B8%3A%20ssl_error_no_cypher_overlap%29%0A» is not allowed to load chrome://global/skin/icons/warning-16.png or link to it. (again home-made translation).

<blockquote> Thunderbird 24.1.1 </blockquote> Update to the latest version (currently 68.5.0). You version is simply too old. https://www.thunderbird.net/

Facepalm... I downloaded new versions of all programs I use before reinstalling Windows, but Thunderbird was the only one I somehow failed to obtain. So I just installed the version from the year 2013 I was using... What could possibly go wrong... :-) Thank you very much again! Everything works fine now. (I have no idea why I didn't mention Thunderbird versin in the first message - something I should have done by default).