While you restore a backup from zenon 5.50 in a higher version as 7.20 (7.60 / (8.x) you get in the editor an error text:
"The docfile has been corrupted."
or, if you setup the editor in German:
"Die Dokumentendatei wurde beschädigt"
Till version 7.11 you can restore the backup. Only in higher versions you get the error-message.
The reason in this case is a corrupt project.prj file. This file must be deleted in an intermediate step.
First, restore your original zenon 5.50 backup file in zenon 6.51. In the next step go to the file-folder from the editor-projectfiles. Now delete the file project.prj. After this, make a new projectbackup in 6.51. This backup can be restored in any higher version.
The error message has to do with "broken" zoom levels (if you want to change the zoom levels in the project properties in the original backup 6.51 version, you get the same error).
After restoring the fixed project backup, please check all project properties, because parameters stored in project.prj are set to default