iMessage asking for password for old email address

Hi All,

I signed out of iMessage directly from the app for an unrelated reason, then tried to sign back in via the normal prompt shown when first opening the app. It pinwheels for a bit, then returns with an 'unknown error occurred'.

I then close iMessage and a system prompt appears asking me to sign in to iMessage (see image) but this prompt contains my old Gmail address which I've not used in several years as my iCloud ID.

If I enter my current account password, it fails of course and states my account using this email address is locked and would I like to unlock it. This is not impacting my current (correct) iCloud account, as I can log in just fine.

I signed out of iCloud on the Mac, signed back in successfully, but this problem persists. I'm not sure what to try next.