TalkTalk are aware of the issue. You don't need to do anything as hopefully it will self correct when a fix is applied.
The message relates to TalkTalk Mail webmail and one of the added secondary email accounts. The error happens because of a failure of re-authentication of the encrypted password.
The mailbox(es) in question are going to be working correctly. So if you wish to view mail messages then the workaround is to sign out of the current mailbox and sign in to the secondary mailbox. Not as convenient obviously as having all mail messages under the one login but needs must.
Or you can have all email accounts added to an email client like Thunderbird. IMAP mail connections are not affected.
