Some OLD interesting info from Revit Clinic - must save here for later!
Cannot Restore Backup Version an Unknown Error OccurredIf when attempting to restore the first version of a backup you receive "An unknown error occurred while accessing xxx.rvt" and then "Failed to extract old document version" the following scenario may apply:
You save a central file for the first time or re-save a project as a central file in a new location. Under Save-As > Project > Options > Preview you specify a source view for the preview image other than Active View/Sheet:
However this view is not open when the project is saved, so no thumbnail is created. Thus in the Revit database, no preview.xxxx.dat is created with the first version of the backup.
Then you later attempt to save a backup version of the project under Collaborate > Restore Backup. You attempt to Save As… the oldest backup version when the central file was originally saved. But you receive An unknown error occurred while accessing xxx.rvt then Failed to extract old document version:
When re-saving or saving a central file for the first time, and specifying a view for the preview other than Active View/Sheet which may not be open when the file is saved, check Regenerate if view/sheet is not up-to-date. This will create a thumbnail for the non-opened view to be included with the initial backup version and eliminate the error message when saving the backup version.
to recover some important data.