That's just a warning. I hope account@gmail.com in the image is not what you've actually entered as your email address. Click Done nonetheless.

Ofc account@gmail.com was just an example on the photo, ive filled this pole with different value (my dad sees espionage everywhere ;D). Problem is still present, this warning appears after i click done, nothing else happening, even after a lot of time, i leave it on. Any ideas? Browsing certificates, changing them/deleting/adding doesn't change anything. The gmail authorisation window also emerges, but after i click "allow", the account is still not added, the warning window is seen again.

re :The problem is not tied with THIS Google Account, emerges all the time, even for different accounts.

Does this mean all the accounts do not work - no incoming and no outgoing ?

It is possible that Thunderbird program is being blocked by the Firewall. Thunderbird must be an allowed program.

If there was a Thunderbird update, it is also possible the Firewall thinks the update is a different program. So check this out first to see if it is causing a problem.

Exit Thunderbird. Access Firewall - this might be the default one installed with OS or it might be controlled by an Anti-Virus program which you installed. Thunderbird must be in the list of allowed programs. If it is not in list then add it. If it is already in list, then remove it or set to block it, then ok settings. Go back into Firewall and reset to allow Thunderbird and ok settings. Start Thunderbird.

Do you use a VPN ? If yes, then switch it off. I use Norton 360 and in an update Norton downloaded and switched on the VPN. It took me a while to realise this as I did not know it had been installed let alone switched on.

re - cookies set as "always turned on"

Confirm you have this in Thunderbird:

• Menu app icon > Preferences > Privacy & Security

Web Content

• Select 'Accept cookies from sites'
• Accept third party cookies: Always
• Keep Until: they expire

But do you actually have any cookies saved - click on 'Show cookies' Do you see, consent.google.com, google.com etc ?

If nothing then cookies are not being stored. Have you created an exception - click on 'Exceptions' Enter this: https://accounts.google.com set to Allow.

Do you have any programs that like to clean up files like CCleaner, WiseCleaner or perhaps an Anti-Virus product that offers it. If yes, then you need to make Thunderbird profile folders exempt. You do not want those types of programs messing around in your profile.

After your tips the problem is still unsolved :( Firewall cheched million times, on this PC any VPN has never been used. Creating exception also did not solve the problem. Im marking issue as solved, no more tries. Thank you for help.