If Jenkins' HTTPS port is 80, Jenkins cannot infer his URI correctly.
Suppose that the correct URI of Jenkins is https://jenkins.local:80/,
he infers that his URI is https://jenkins.local/.
It lacks the port number 80.

How to reproduce:

• Install Ubuntu 11.04 Server 64bit
• Install Jenkins using apt
• Edit /etc/default/jenkins as following:
  • JENKINS_USER=root
  • JENKINS_ARGS="--webroot=/var/run/jenkins/war --httpsPort=80"
• Restart Jenkins. service jenkins restart
• Go to https://jenkins.local:80/configure
• If Jenkins URL is not empty, make it empty and save configuration
• Go to https://jenkins.local:80/configure again
• Jenkins URL on that page is https://jenkins.local/

A workaround is to set Jenkins URL explicitly.