For others who are also having the same issue, but it was not solved by deleting/re-downloading the site - I posted a very strange alternative solution here:
I have a solution that actually works but you need to be very quick. Step 4 is where the magic happens.
Add the site as normal in DevKinsta and wait for it throw the error DK0066.
Open the backup.sql file in a fast text editor like Sublime. Some IDE’s might be a bit too slow.
Delete the first line /*!999999- enable the sandbox mode */ and Save.
NOW is the tricky bit, which nobody seems to have mentioned. Read all of this step first before starting. You need to click ‘Retry’ in DevKinsta which …