Thunderbird account set up puzzle

[#1]

I have set up most of my accounts with own inboxes. But one is giving me problems. Server settings, user name etcetera are OK and accepted by system. However my password, the same as I have used before and in similar format to those of other accounts is continually rejected once the account is selected for creation.
Suggestions please why on earth this is happening and a cure!

[#2]

Are you able to login to the problem account using webmail?

[#3]

All log in OK in webmail

[#4]

OK, that shows the mailbox works properly.
In Thunderbird, click Tools and select Options, then click Security and the Passwords tab.
Click the Saved Passwords button to see a list of your accounts.
Click Show Passwords and confirm (Yes).
You can now compare User names and associated Passwords.
Is there a line for the account which fails the creation process?
If there is you could try highlighting the line and click Remove to delete it. Then try creation again.

[#5]

O Moderati populusque! Mea culpa! Praesent euismod gravis senior  erat.
and for normal mortals
it was a chronic senior moment.