Hello Philip,
The certificate says issued to localhost, however I doubt in the browser you entered https://localhost:50001/irj.
So the certificate should not be issued to localhost.
You may want to check this KBA: 1770585 - How to configure SSL on the AS Java.
And at the end the browser also needs the SAP root certificate installed in the certificate store (the install certificate button in the picture does the trick).
Best Regards,
Peter