When performing a migration operation, the following message is displayed:

The object with the following address '{0}' was expected to be located under the following web address '{1}'. This can happen when there is an error in the Alternate Access Mappings configuration.

This error means that ShareGate Migrate expects the address of an element to be located under another one, but it is not the case. For example, a custom form for a list should normally be located in a folder within the list's root folder, so it should be located under the list. If the list's address is "http://site/list", ShareGate Migrate expects the form's address to start with "http://site/list/...". If the form's address is something else, like "http://site/anotherList/..." or "http://anotherSite/list/...", ShareGate Migrate will give this error.

This error is often related with forms or workflows, and the most common cause is when the form or workflow was copied or moved from somewhere else, without being updated correctly.

This issue can also happen because of the Alternate Access Mappings configuration. For instance, if you create "Site Name" Site Collection, you must add it to the Alternate Access Mappings. For more information on how to manage Alternate Access Mappings, please see here.
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The first thing to verify is if the object is at the correct location on your source site. In order to do this, connect to your source site with SharePoint Designer and click on "All Files" to navigate within your site's folder structure. Then, locate both the parent element (normally the site or the list) as well as the problematic element (most often a form or a workflow file) and verify if the problematic element is correctly located within the site/list. If it is not the case, try to copy it or to create a new one at the right location, and then update the site/list to refer to the new element instead of the old one.

If everything seems to be constructed correctly, then the issue might be with the Alternate Access Mappings configuration. You can read more information about the way they must be configured here.
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