The problem is most likely caused by a partial download of the projects. When downloading a project for the first time in Paratext 8, we request batches of updates until the whole project is received. If this fails part way through, we expected the next send/receive to complete the process, but we didn’t take into account that we may not have received enough of the project history to include the migration to Paratext 8. This means that the project is still a Paratext 7 formatted project and can’t be opened in Paratext 8.
You will need to delete the project folder and retry the download. If the projects are large and you can’t download them, send we an email and I’ll create a .zip file for you.
We are upgrading the version of Mercurial that we use in Paratext 9 which allows us to do the initial project download in a different way - we won’t have the partial project problem.
John+Wickberg
Paratext Support