I started to notice that Azure AD connect was failing and had a look in the console and seen the error message in the console displayed in the image below
Then when I tried to run the command Start-ADSyncSyncCycle -PolicyType delta , I received a long list of authentication errors.
To resolved the issue browse into properties of the Azure Active Directory account and copy the account name onto the clipboard.
Next add this account in as an excluded account from conditional access policies the same way a break glass cloud identity admin account would be excluded from conditional access policies.
As soon as the sync account has been added to the exclusions , syncs will resume