So the DOB should come in as a string, removing the quotes will create an invalid JSON.
This is the line that does the DOB validator
Just eyeballing it, I can’t see what’s wrong with your date format. You may have to put a breakpoint in on a local box and try to step through the code and figure out what is going on as from what I can see it looks like “2023-01-30” should work just fine for the DOB.
So are you able to register a client on the OpenEMR demo servers?
Have you checked that your apache mod_rewrite is enabled (which is what is used to map the /oauth2/default/registration to the /oauth2/authorize.php file.
Is Postman able to hit the main OpenEMR login page and retrieve a result?
If you have something time sensitive you may want to consider reaching out to an OpenEMR vendor to pay someone to expedite resolving your issues.