Nothing? No one knows how to fix this problem? My problem is worse now. Each time I click a subfolder, I immediately get the error message and TB will not display the contents of the sub folder. Help, please.

When you select any imap folder there is communication between Thunderbird and the server . Synchronisation occurs to ensure the imap folder is updated and displaying exactly the same as seen on the server in folder of same name.

The message seems to state that Authentication failure is occuring when attempting to synchronise the folder.

Does this only occur when you select sub folders or do you also get this problem when selecting any default folder like the 'Inbox'?

Please check to see if all your folders are 'subscribed'. Right click on the imap mail account name in Folder Pane and select 'Subscribe' click on 'Refresh' Select all the folders and subfolders in the list and click on 'Subscribe' click on 'OK'

Did you see all the folders and subfolders in the 'Subscribe' list ?

Logon to the webmail account via a browser. Do you see all the same folders and subfolders as you see in Thunderbird imap account and do they contain emails ?

I followed your first reply ok. Yes, I see all the folders and subfolders in the Subscribe list. I do not understand your instructions in your second reply. How do I "Logon to the webmail account via a browser."?

re :How do I "Logon to the webmail account via a browser."?

A browser is the program you use to look for websites etc on the internet. Such as Firefox, Edge, Chrome. You mention AOL, so you would be using AOL website to access emails. When you logon you use your email address and the password you set up. Try this link https://login.aol.com

I understand how to use a browser. You say "Logon to the webmail account" - what "webmail account"? Do you mean log on to AOL? You say "you would be using AOL website to access emails" No, I do not use the AOL website to access emails, I use TB. I guess since TB is accessing AOL, then I am accessing AOL in a second-hand kind of way.

I opened Chrome, opened and signed on to my AOL account. It's difficult to determine because the structure is very different but I think all the folders are there. The big difference is that the folders on the AOL web site do not have any subfolders attached. The subfolders are listed individually as though they were folders also. All folders on the AOL web site are listed alphabetically. While difficult to determine, I think all the subfolders are there just not attached to the parent folder.

re :what "webmail account"? Do you mean log on to AOL? You say "you would be using AOL website to access emails" No, I do not use the AOL website to access emails,

Yes logon to AOL to see the server folder - this is your webmail account. I meant because you are using an aol server, you will need to use the aol website to access the emails.

re: I opened Chrome, opened and signed on to my AOL account. That's correct.

re :The big difference is that the folders on the AOL web site do not have any subfolders attached. The subfolders are listed individually as though they were folders also. All folders on the AOL web site are listed alphabetically. While difficult to determine, I think all the subfolders are there just not attached to the parent folder.

In the AOL webmail account One of your top level folders should contain specific subfolders, but it does not because they are listed as ordinary top level folders. I've tried to locate some info on how to move a folder into another folder to convert it into a subfolder. It seems AOL does not provide such a feature.

I do not know what you see in the Thunderbird IMAP account as I have no idea what your folders are called, but as an example senario let's say you have a top level folder called 'Hobbies' and it has two subfolders called 'Football' and 'Rugby'.

In the AOL webmail account- as an example - you would see all three folders 'Hobbies', 'Football' and 'Rugby', but they are all top level folders.

It seems you have to have to create a new subfolder and then move the emails from one folder to another. So in the example senario: Create a subfolder in the 'Hobbies' folder and call it 'Football' See info at this link on how to create a subfolder: https://help.aol.co.uk/articles/create-delete-or-rename-folders-in-new-aol-mail

Then open the original 'Football' folder to see all the emails. Now you need to select those emails and use the Move to and select the subfolder called 'Football' See link on 'Move emails to another folder' https://help.aol.com/articles/move-emails-into-folders-in-new-aol-mail

Once the emails have been moved from the original 'Football' folder into the new 'Football' subfolder, you can remove the original now empty 'Football' folder.

Leave the webmail logged in and go to Thunderbird and select the equivalent subfolder - whatever it was called which you created in AOL webmail. It should synchronise because there is now a subfolder of same in AOL. It should get all the emails to display.

If that works then go back to the webmail account and perform the same action to get new subfolders of same name and move emails as before.