Prelude:

I'm running Jenkins inside a docker container, behind an Nginx (running in other container)

Jenkins respond to http://<serverNoPort>/jenkins port 8080 is not exposed.

Found 2 related issues already closed but without solution: JENKINS-39238 and JENKINS-38957

I hope I provide more information to reproduce

Steps:

1. Create a new item
2. Try to add new credentials (example: source code gitHub)
3. Fill the credentials and save. 

Error: 

It fails silently (user doesn't get any error on screen) but in the browser console the following message is found

Failed to load resource: net::ERR_CONNECTION_TIMED_OUT
select.js:156 Uncaught DOMException: Blocked a frame with origin "http://<serverNoPort>" from accessing a cross-origin frame.
{{ at HTMLIFrameElement.<anonymous> (http://<serverNoPort>/jenkins/adjuncts/69ad1dce/lib/credentials/select/select.js:156:57)}}
(anonymous) @ select.js:156

http://<serverNoPort>:8080/jenkins/descriptor/com.cloudbees.plugins.credentials.CredentialsSelectHelper/resolver/com.cloudbees.plugins.credentials.CredentialsSelectHelper$SystemContextResolver/provider/com.cloudbees.plugins.credentials.SystemCredentialsProvider$ProviderImpl/context/jenkins/addCredentials

It seems that it is trying to reach the Jenkins server trough the default port (that is not exposed) and subsequently fails.