tanstaafl wrote:"2 accounts my wife & I but TBird only accepts mine now & It rejects any attempt with "Incoming server already exists". There are no messages ZERO no old & new."
If you each have a Gmail IMAP account why are you trying to add a account? It sounds like you are having a problem with your inbox and your reaction is to try to add the account again (and failing because its a duplicate of an existing account).
If you select another folder than the inbox can you see messages in the folder listing?
Can you open and read one of the old messages in a folder other than the inbox?
As a sanity check that your username/password is still good by logging into Gmail webmail using a browser (if you haven't already done that) and check that you can see some messages in the inbox. I assume you're not configured for two step verification (i.e. you didn't have to create a app. password to use instead of your accounts password)
Diagnostic: "Unable to login at server, Probably wrong configuration username or password"