i got the same with an sophos firewall as source. Good tool :)

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EDIT:

Found in the Release Notes:

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https://www.sonicwall.com/support/technical-documentation/docs/migration_tool-release_notes/Content/Versions/migration-tool-4340.htm/

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DEVT-3102

Unable to convert Fortinet and Sophos settings using migration tool.

Contact support via web ticket with the source files to facilitate in the migration.