Ironically I'm looking at a solution where I get a 404 when I try clicking the Migrate-button to "Migrate redirects from DDS to SQL" in the Administer-section of the Custom Redirects Manager:
The post looks like this:
curl 'https://www.mysite.com/episerver/BVNetwork.404Handler/NotFoundRedirect/MigrateDdsToSql' --data-raw 'gadgetId=4b5675a4-2c67-46e0-9ae4-1e205ae62068'
Will dig further but feel free to tell me if you have seen this...
The Migrate-button works in other environments for the same solution.