This issue is generally encountered when moving forms from one site to another or when restoring old back-ups. This is generated by the fact that the version in which the back-up was created is not the same as the one you are trying to restore the forms in.If you do encounter this issue, then please try:
- accessing your former installation and update it to the latest version of the component;
- generate the backup file one more time;
- make sure that the latest version of the component is used in the second installation as well;
- restore the backup.
If you have multiple large forms or forms that have a large number of submissions then it would be best to create one back-up for each form separately.