Calendar: domain DAAccountValidationDomain / error 100

I have bought a iMac Mini M4 and have been setting it all up. However, my Calendar App is not fully syncing, it is showing some of the emails but not my default eamil, even though it is all synced and okay with several emails linked to it on my iPhone and iPad. I am getting the above error on my default email, even though the email is not showing it just says iCloud. Please advise.