This is a result of a change in FH18.104.22.168:
If you run Validate and allow FH to correct the issues, your FH6 sort dates will be converted into Media records.If you open a project that contains embedded images or local media objects, you will be prompted to run Validate (e.g. File > Validate). The Validate process now checks for media issues and has an extra option to correct media issues found. ‘Media issues’ means local media objects or embedded media. The Validate process will report on both. If the option to correct media issues is ticked, local media objects will be converted to use media records, and embedded media will be converted to project media files.
I recommend you do not select Validate but open the file without validation and download and run the latest version of 'Order Facts by Sort Date' which will copy your 'Mike Tate'-style sort dates into FH7 sort dates.
You'll still get the Validation error next time you start up, because the latest version of the plugin doesn't delete the old-style sort dates. If you accept the prompt to validate this time (assuming you have no use for the old style sort dates) you need to go to the Media Records List, sort it descending by Record ID, select all the 'Sort Date' media items and add them to an empty Named List, then select the named list, right click and Delete Listed Records.
Mike, perhaps you should incorporate some or all of this advice into Help for the plugin?