Similar here: a credentialsDialog emerges in my https:// configuration page, rendering a form whose POST URL is http://<servername> instead of https://.

Steps:

1. Create GitHub Organization or Multibranch Pipeline job (for this example, I'm creating a GitHub Organization job named kkk)
2. Projects > GitHub Organization > Scan credentials > Add kkk (Folder Credentials Provider)
   1. jenkins gets the dialog: GET https://<jenkins_server_noport>/descriptor/com.cloudbees.plugins.credentials.CredentialsSelectHelper/resolver/com.cloudbees.plugins.credentials.CredentialsSelectHelper$ItemContextResolver/provider/com.cloudbees.hudson.plugins.folder.properties.FolderCredentialsProvider/context/kkk/dialog
   2. the response already contains the POST to the incorrect URL:

 <div class="hd"><h2 tooltip="Folder Credentials Provider"><img src="/static/85cead7b/plugin/credentials/images/48x48/folder-store.png" style="width: 48px; height: 48px; " class="icon-credentials-folder-store icon-xlg" /> Folder Credentials Provider: kkk
 </h2></div><div class="bd"><h3><img src="/static/85cead7b/plugin/credentials/images/24x24/new-credential.png" style="width: 24px; height: 24px; " class="icon-credentials-new-credential icon-md" />
 Add Credentials
 </h3><form method="POST" action="http://<jenkins_server_noport>/descriptor/com.cloudbees.plugins.credentials.CredentialsSelectHelper/resolver/com.cloudbees.plugins.credentials.CredentialsSelectHelper$ItemContextResolver/provider/com.cloudbees.hudson.plugins.folder.properties.FolderCredentialsProvider/context/kkk/addCredentials" id="credentials-dialog-form"><!--omited--></form></div>

I have recursively grepped through all jenkins config + data dirs, but couldn't find any explicit reference to that url http://<jenkins_server_noport>.

Environment:

• No docker involved
• Jenkins listens http in 8080 (firewalled)
• Jenkins is front-ended with an nginx on corresponding https://
• Manage Jenkins > Configure Jenkins > Jenkins Location > Jenkins URL = "https://<jenkins_server_noport>/"
• Jenkins v2.55
• Plugins:
  • cloudbees-folder v6.0.3
  • credentials v2.1.13
  • github-branch-source v2.0.5
  • github-organization-folder v1.6

By the way, I can work-around this:

• Use a browser with editing capabilities and edit the POST URL 'on the fly' changing the http:// with the correct https://
• Fill in the form and send it

Are you injecting the  X-Forwarded-Proto header nginix to ensure that Jenkins knows the request URL is over https?

In my case yes.

My problem is the 8080 port that isn't exposed in my server as all the http/https requests must go through ports 80/443 and through the nginx.

Right so if you are changing port (and perhaps host too) then you would also need to inject X-Forwarded-Port (and perhaps X-Forwarded-Host too) so that Jenkins has the information to reconstruct the request url.

Until you have set that up correctly, this is not a bug but rather a configuration issue on your end (note that the core Admin monitor should be giving you a warning when you go to the "Manage Jenkins" screen if you have not got your reverse proxy configured correctly, so if that is not happening then that might be a bug against Jenkins core, but this is not sounding like a bug against credentials plugin)

Thank you for your help. 

I think now is working. It was the X-Forwarded-Port. 

Adding the X-Forwarded-Proto also solved the problem for my. Thanks a lot!