Note: Adding the OP’s solution from the question as an answer.
This is what worked for me. deleted all localhost certificates. ran repair IIS Express in Add/Remove Programs. Copy and pasted localhost certificate from Personal/Certificate to Trusted Root Certificates and it didn’t complain anymore.
Repairing/reinstalling IIS Express does not appear to actually fix this issue (as of IIS Express 10), the real solution is to add the localhost certificate to Trusted Rood Certificates.
Full details for how to do this is covered by this blog post: https://blogs.iis.net/robert_mcmurray/how-to-trust-the-iis-express-self-signed-certificate
In summary,
- Open the certificate manager via “manage computer certificates” in control panel or by running certlm in a cmd prompt
- Under Personal>Certificates export the localhost cert and save it to disk
- don’t export the private key! Just export the public key.
- Under Trusted Root Certification Authorities, right click on Certificates and import the cert you just exported