My process has always been to omit the password while setting up account. Thunderbird does not need that to validate the account. On first retrieval of messages, Google should prompt for password. Enter the password used online and Google should then update TB with a unique password.

Many thanks, David

I omitted to say that the online password didn't work either. That's why I obtained an App password from Google.

I've tried again this morning to add the existing account to Thunderbird - which recognises the configuration etc - but confirming the correct email address takes me straight to the same error message. No opportunity to enter either password!

Might something have become corrupted? Would my sharing the relevant 'code' / web address assist diagnosis?

Thanks again.

You are confusing me. Are you saying you cannot login online either? In my experience, one does not first obtain an app password, but lets Google provide during account setup. I can't help, other than to advise what works for me: define account info without password to complete setup. Then respond to Google prompt with the online password.

I can log in online without a hitch.

When trying to set up the account on Thunderbird, first I used my usual online password. When that didn't work I tried an App password instead, but to no avail.

Yesterday the Google prompt was asking for confirmation of the email address followed by a password. This morning when I confirmed the address, it took me straight to the error message, without opportunity to enter a password. Now its not recognising the email address it has just accepted in the previous stage of the process. And when I ask it to 'find my account', it provides information about allowing cookies (not needed as all relevant exceptions are already in place).

Many thanks again for your time and attention. The mystery remains to be solved.

Have you tried setting it up with no password?

Yes - thanks for the tip, but it doesn't solve the problem in this case.

Do you get an error? Does the account already exist on Thunderbird?

For a long time now I've been using the account on Thunderbird without a hitch. Then yesterday, when TB tried to access messages, a Google prompt said to log in.

Several attempts to do so plus all relevant checks having failed, I then removed the account from TB to see if that cleared the system.

Now it won't let me reinstate the account on TB.

I get this error message

"400. That’s an error. The server cannot process the request because it is malformed. It should not be retried. That’s all we know."

Or it just doesn't recognise the correct address for an email account that's working fine online and invites me to enter another address.

Have you deleted all the saved passwords for the account and oauth tokens for that account from the password manager?

How I wish folk would not jump to remove and re add when it is not working. I am not aware of anyone where it has worked and fixed a problem where the problem was not their choice of security software.

Yes, I have deleted all relevant passwords and tokens, thank you.

When I try to add a brand new gmail account to Thunderbird for the very first time, exactly the same thing happens.

Problem solved... ...by simply creating/switching to a different profile (from 'Help' go to 'Troubleshooting Information', then click on the 'aboutprofiles' link)

Gmail accounts old and new now added to Thunderbird without a hitch

Many thanks again for all assistance proffered

I created multiple new profiles within Thunderbird. It didn't work. I get the same error. It is not at all clear what creating a profile even does within Thunderbird.

So, what I'm trying to say is that the supposed solution of "creating a new profile" didn't work. As far as I can tell, it does not do anything.

1. Why are there default and default-release profiles? 2. Why are each of them listed in AppData\local and Appdata\Roaming? 3. Should there be these two profiles in two places? 4. What is the relation between profiles and the gmail "The server cannot process the request because it is malformed. It should not be retried. That’s all we know." error when entering a otherwise functional and error-free gmail account into a new Thunderbird install having other gmail account(s)?

1. Why are there default and default-release profiles?

https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation

2. Why are each of them listed in AppData\local and Appdata\Roaming?

Perhaps understand what the local, locallow and roaming folders purpose in Windows are and the answer becomes rather self evident. The roaming folder is for data that is needed should the windows user account/profile be moved to another windows machine using windows roaming profiles. In the case of Thunderbird the bulk of your mail and settings data is required for the application to run seamlessly and the same as on the last machine you logged into. Some data like the cache is not required and is stored in the local folder.

3. Should there be these two profiles in two places?

That depends. Most folks never use windows roaming profiles on a local network, so it is not important to them at all. But having them certainly does not have a downside for those folks and it does have an upside for those that use windows roaming profiles.

4. What is the relation between profiles and the gmail "The server cannot process the request because it is malformed. It should not be retried. That’s all we know." error when entering a otherwise functional and error-free gmail account into a new Thunderbird install having other gmail account(s)?

Nothing.

If you want support please post your own topic. as this topic has generated three off topic posts since it was solved, I will now lock it to prevent further issues.