Make sure the attributes used as auxiliary attributes for migration/synchronization are not used and do not contain information (are not populated). Such attributes in target domain will be overwritten.
Populated attributes in source domain will cause an error and will be discarded (in a one way synchronization). The warning is informational and - if not causing inconveniences - can be (in most cases, see below) ignored.
However- if you are using an LDAP filter in the source scope to only synchronize certain objects, and it is based on this attribute, the object will not be synchronized. In this example, the filter would be (wWWHomePage=migrate).
NOTE: If the synchronization is configured to be a two way synchronization, then DSA will populate the auxiliaryand matching attributes for source and target objects.