hello!It sounds like there are two separate issues here:

   One Gmail account works, but the other fails with a "400 ... malformed" error.
   Your old emails and files are missing in Thunderbird after reinstalling your OS.

Let's tackle them one by one. Issue 1: "400 ... malformed" Error When Adding the Second Gmail Account

This error typically means that Thunderbird is struggling with Google’s OAuth authentication. Try the following:

✅ Step 1: Enable IMAP for the account

   Log in to your Gmail via a web browser.
   Go to Settings (⚙) > See all settings > Forwarding and POP/IMAP.
   Ensure IMAP is enabled.

✅ Step 2: Check Less Secure App Access (or Use App Passwords)

   Go to Google Security Settings.
   If 2-Step Verification is enabled, generate an App Password for Thunderbird and use it instead of your regular password.
   If 2-Step Verification is off, try enabling "Less secure apps access" (Google has removed this option for many accounts, so App Passwords are preferred).

✅ Step 3: Remove Any Broken Gmail Tokens in Thunderbird

   In Thunderbird, go to Preferences (or Options) > Privacy & Security.
   Scroll down to Saved Passwords > Remove Saved Logins for the failing Gmail account.
   Restart Thunderbird and try adding the account again.

✅ Step 4: Manually Add the Account in Thunderbird

   Go to: File > New > Existing Mail Account.
   Enter your details (email and password).
   Manually enter IMAP/SMTP settings if automatic setup fails:
       IMAP Server: imap.gmail.com | Port: 993 | SSL: SSL/TLS | Authentication: OAuth2
       SMTP Server: smtp.gmail.com | Port: 465 or 587 | SSL: SSL/TLS | Authentication: OAuth2

Issue 2: Missing Old Emails and Files After OS Reinstallation

Thunderbird stores emails locally in a profile folder. If you didn’t back up your old profile, the emails might be lost unless they were stored on the Gmail server (IMAP).

✅ Step 1: Check If Your Emails Are Still on Gmail

   Log in to your Gmail account via a web browser and see if your old emails are still there.
   If they are, they will resync once IMAP is set up correctly.

✅ Step 2: Restore the Old Thunderbird Profile (if backed up)

   Find your old profile backup (look for a folder named xxxxxxxx.default).
   Locate Thunderbird's current profile folder:
       Windows: C:\Users\YourName\AppData\Roaming\Thunderbird\Profiles\
       Linux: ~/.thunderbird/
   Replace the new profile with the old one (after backing up the new one).

✅ Step 3: Check Thunderbird Import Options

   If you backed up your profile, but importing fails, try using the Thunderbird import tool:
       Tools > Import > Import from another Thunderbird installation
       Try selecting the backup file manually.

Final Thoughts

   If one Gmail account works and the other doesn’t, the problem is likely authentication-related (OAuth, App Passwords, or IMAP settings).
   If old emails are missing, check if they were stored locally or if they still exist on Gmail.
   If you backed up your Thunderbird profile, manually restore it to recover old emails.

Let me know if you need more details! 🚀With regards Sakura

Not sure that's the problem, but you have to set up the Gmail accounts in Thunderbird as described here:

https://support.mozilla.org/kb/automatic-conversion-google-mail-accounts-oauth20