Do you have cookies allowed in Edit/Preferences/Privacy & Security, and is there some external 'cleanup' app that acts on the TB profile folder and possibly deletes cookies?

Is OAuth2 set on both the incoming and outgoing servers?

@sfhowes:

I have cookies allowed "until they expire".

I did not install a "cleanup" program, and I don't think KUBUNTU has one as a defult. At any rate, the list of cookies is long, so there was no deletion.

The OAuth method is used both on IMAP and SMTP servers.

I should say that this problem is quite new. Only the last few months (I can locate the date it started by digging in my files.) For several years the combination TB + GMAIL worked very well for me.

It looks like everything is set up properly. Since it works intermittently, it suggests there's some external app or hardware causing the failure. Have you started using a VPN recently? It may seem simplistic, but removing the passwords and oauth tokens from Saved Passwords in Edit/Preferences, then restarting TB and entering the account password in the oauth window, might reset the connection.

@sfhowes: No, there is no new VPN, or any VPN at all, except for short duarations.

I have tried removing passwords, and even deleting the GMAIL account altogether, in a few sequences and variations.

Nothing seems to work.

Even right now I could not read IMAP, but after quitting TB and restarting I got the connection and was able to open this email that I am now responding to.

At times of urgency I find myself opening gmail on a browser. Or using the android app aquamail (on my phone), that never has any trouble connecting to gmail.

Create a new profile and add the gmail account. Help/Troubleshooting, about:profiles, to create and launch profiles. If the problem remains in the new profile, you can probably conclude the cause lies outside TB. If the new profile works, mail and contacts etc. can be transferred from the old profile to the new.

I do have a wifi extender in my home, and tried to see if disconnecting it makes a difference.

The result of the experiment, since yesterday, is positive - it looks like this extender (TPLINK RE200) caused the trouble. Does this make sense to you?

There have been cases where a user enters custom settings in a router, that affects TB performance, or the properties such as iPv4 vs. iPv6 in a router conflict with the capabilities of the ISP. I can't say exactly why the extender causes the problem, but I'm not too surprised.

Sadly my happiness was premature. As I write TB is again stuck, and I had to read this notification email via gmail in a browser... This while the extender is disconncted.

In options/preferences try setting Thunderbird to no proxy. I have seen a couple of instances where this fixed random connectivity issues on OSX, given both operating systems are unix based it might work on Linux.

@Matt: Thanks for the tip. Will try it and report back.

@matt: Since making this change in preferences (no proxy) two days ago, both my ubuntu computers did not have a single occurrence of this gmail error.

I will report again in a few days.

But it seems that this is a solution to the problem!

This minor change - "no proxy" - solved my problem. It's now 6 days since I made this change, and I did not have a single instance of authenticcation problem.

@matt: You might want to poat this solution in other related places.

Update: today the authentication problem is back.

I wonder if this is related to yesterday's crash at GOOGLE, apparently about pwd auth? Maybe they made some stricter rules after that?

same problem. the "proxy" settings have no effect anymore.

I have an on again off again internet connection that has something to do with my netgear router. It appears the DNS resolver is not passing on requests at times. Or my service provider has a poor DNS server I keep getting.

I made this significantly better by using dns over HTTP. The settings for firefox and Thunderbird are the same, but you use the config editor to edit the settings in Thunderbird

https://wiki.mozilla.org/Trusted_Recursive_Resolver#network.trr.mode

Again just another guess.

I am trying this. Changed to

network.trr.mode = 2 

Is this the correct setting?

On my other computer (where I did not touch the trr setting) I noticed the following interesting thing today (not sure how consistent it is): when connected to gmail via the firefox browser, the nonresponsive thunderbird -> gmail connection seems to connect.

Does this make any sense?