Update results in failed DB check
Are you sure the problem isn't already on the "old" site?

Could you try to (without changing any code) go to the civicrm upgrade page on the old site (/civicrm/upgrade). do not upgrade, just look at what's on the screen

Do you have a message saying you are already at the correct version and nothing to be done?

Sorry but we are not able to provide this at the moment.
Your database has already been upgraded to CiviCRM [your version]


... or do you already have a message saying database check failed?

If the later, it means that you had a partial update already that was never properly sorted. It's recoverable, but you might have to dig into sql more than you'd like (or hire an expert to help you, probably an easier option to help you untangle the situation)