When I try to add credentials for example login/password for docker registry, nothing happens and I get following error in console (domain name changed to example.com):

 Mixed Content: The page at 'https://example.com/job/dfo-pm-staging_backend_develop/configure' was loaded over HTTPS, but requested an insecure form action 'http://example.com/descriptor/com.cloudbees.plugins.credentials.Cred...ials.SystemCredentialsProvider$ProviderImpl/context/jenkins/addCredentials'. This request has been blocked; the content must be served over HTTPS.

I can change form url to https instead of http using developer tools and then it submits correctly. How do I fix this issue?