SetOwner could not convert "" to a SID (The security ID structure is invalid) appears in the file migration log. This usually occurs with 'Matched' objects
When you do a user match instead of a migration the SID is not updated in the database.
Before you run a file migration, Run a file scan to update the SID's in the database.
You will see lines similar to the following when it detects a user with no SID's at the top of the log file before the actual file scan begins:
16 - [09:56:45.981] Updating database object CN=User1,OU=Test,DC=Quest,DC=local with AD SID S-1-5-21-0000000000-0000000000-000000000-0000