When migrating SharePoint Site Collections with On Demand Migration (ODM), you may encounter a warning similar to the following:
An issue occurred while processing web parts for ...User mapping is not found. Source user:
When you encounter this warning, you may want to compare the source and target Site Page referenced side by side. This warning does not indicate that the important data was not migrated. This warning was likely triggered by an element that specifically identifies the user that was not matched.
To resolve this warning, the source user would need to be matched to a target user. If the source user will not be migrating to the target, then this warning may need to be ignored.
