Whenever you sync an on-premises server with Azure active directory, there is a potential for something to go wrong during that syncing process. Should this occur, it's best to refer to Microsoft's documentation which I've linked below. However, let's go over some of the more common synchronization errors and how you might fix them. These errors can generally be broken down into categories each with their own unique errors, and these categories are; data mismatch errors, duplicate attribute errors, data validation errors, large object errors, and existing admin role conflict errors. One of the more common data mismatch errors is the invalidsoftmatch error. This occurs when a sync is attempted and the incoming objects attributes don't properly match with the attributes of the objects already in Azure AD. Specifically, when adding or updating an object, Azure AD looks for the incoming objects "SourceAnchor" attribute and matches that to the "ImmutableID" attribute of the existing attribute in Azure AD. This is what is referred to as a hard match.

If the hard match fails, then before it attempts to provision a new object, it attempts to utilize the "ProxyAddress" and the "UserPrincipleName" attributes, and if it successfully finds a match in this way, it is known as a soft match. When it fails to find a hard match and then successfully finds a soft match, but the attributes used in the hard match check are different than what the soft match attributes would suggest, this creates the InvalidSoftMatch error. In order to fix this error, the administrator should identify the duplicated attribute causing it. You can find the source of the errors within Azure AD Connect health report,  but the most common culprits are the "ProxyAddresses" and the "UserPrincipleName" attributes. 

Determine which should maintain their value and which should not, and then remove that incorrect value and change it in the directory where that object is sourced. Another common data mismatch error is the ObjectTypeMismatch error. This type of error occurs when Azure AD attempts to soft match two objects that are different object types. For example, you may have an existing Microsoft 365 group labeled as HR@Macaroni.com. However, your on-premises directory may have a user with their proxy address as HR@Macaroni.com. When Azure AD Connect attempts to synchronize this, it will realize that one object is a group and one object is a user, and throw back the ObjectTypeMismatch error.

In order to fix this, we follow the same process as the last error. Identify the duplicate objects, determine which should remain and which should change or be removed, and then re-attempt the sync. Both of these errors are at their core similar to duplicate attribute errors, which pretty much tells you what these next errors are about. Azure AD doesn't allow two or more objects to have the same value for certain attributes. These attributes are; the mail attribute, the proxyAddress attribute, the signInName attribute, and the userPrincipleName attribute. 

When Azure AD Connect attempts to sync an object when an object already holds the same unique attribute, it will throw back the AttributeValueMustBeUnique error. This error can be fixed by following the exact same procedure as the previous two errors. Identify the duplicate object, determine which should remain and which should change or be removed, and re-attempt the sync. Although it's worth mentioning that while Duplicate Attribute resiliency wouldn't fix the error, it would at least allow for the sync process to complete while you figure out where the issue is and how to resolve it.

For more information on Duplicate Attribute resiliency, check the documentation below. The next most common errors would be the DataValidation errors. These are as simple as ensuring that you're utilizing supported formats and characters and will throw back the IdentityDataValidationFailed error. Specifically, admins need to ensure that the UserPrincipleName attribute is properly formatted and is using supported characters. Similar to the IdentityDataValidationFailed error, we have the LargeObject error. 

Simply put, this is thrown when one of the following attributes exceeds its size, length, or count limit. The userCertificate attribute, the userSMIMECertificate attribute, the thumbnailPhoto attribute, or the proxyAddresses attribute. In order to fix these errors, you find the attribute causing the error and remove values that are no longer required, like expired certificates or outdated addresses. And finally, the last error we'll be covering is the Existing Admin Role Conflict error. This occurs when Azure AD Connect attempts to sync a user object from its on-premises AD with the user object in Azure AD that is assigned an administrative role.

Azure AD doesn't allow syncing of user objects when the Azure AD user object has an administrative role assigned to it. In order to fix this error, you must first remove the administrator roles from the user in Azure AD and then delete the quarantined objects in the cloud. Once completed, the user object should sync properly with the admin roles removed. After the sync has been completed, you can then go back in and assign the proper administrative roles to that user in Microsoft 365.