Can't add Google Drive account

[rudyz]

Hello,
I am unable to add my Google Drive account to FreeFileSync. Everytime I try it, I select my account in browser it opens, agree with giving permissions, but then I get error message:

Unable to connect to Google Drive.
ENOTCONN: Transport endpoint is not connected [shutdown]

I tried turning off my firewall and choosing my second Google Drive account but it makes no difference. I am not using any VPN.

Thanks for any suggestions in advance.

EDIT: I use Linux Mint 20 Xfce.

[Zenju]

Which browser are you using? Are you seeing the "Authentication completed" page in the browser? If not what's the last thing you see in the browser?

[rudyz]

I was using Firefox. I did not receive "Authentication completed" message after giving permissions, instead a generic Firefox error screen with message:
Unable to connect
Firefox can’t establish a connection to the server at 127.0.0.1:56621.

After your suggestion that browser might be to blame I tried using different browser, Google Chrome. I switched my default browser in settings to Chrome and tried same routine using Chrome. This time, I managed to successfully add my Google Drive account to FreeFileSync. Everything seems to be working well even after I switched my default browser back to Firefox (I started a task to backup files to Google Drive).

Thanks a lot for your help!