macbook pro m1 imessage An unknown error occurred

HI,

New MacBook Pro M1 to replace 2014 iMac. AppleID and pwd work online/web, in Music on iPhone and iPad. iMessage returns 'An unknown error occurred'. Tried what I could, from the logging out from the old iMac, log in to different local user account then back to my user account..... to removal of a few plist files as suggested in a developer post to no avail.

What else can be tried?

Thanks in advance for any help!!