If you received the following message:
"Sorry but this form response cannot be validated. We detected that your spreadsheet is corrupted, please click on this link and start Form Workflow Plus add-on so we can correct the error for you. Once done refresh this page and submit your validation again"
it means that Form Workflow Plus technical columns were not found in the spreadsheet or they were deleted accidentally by the user. We have to run the add-on again to fix this.