After updating both Jenkins and all plugins, I notice that the added links on my sidebar are missing their icon. My icons are stored in Jenkins' user content folder, and 'Link Icon' was set to /userContent/Jenkins-extra-link.png (so a link relative to the current site name, to make sure the link works with all the url variations possible with an 'internal' dns name, so the machine can be accessed under e.g. http://machine and http://machine.company.com). This used to produce a <img src="/userContent/Jenkins-extra-link.png"> (just what I entered) and that worked. Now it ends up doing <img src="//userContent/Jenkins-extra-link.png"> and that breaks.

I see the "Copy Multi-configuration project" link (make by the "Matrix" or "Matrix reloaded" plugin I think) suffers the same issue:
src="//plugin/copy-project-link/img/edit-copy.png" so this may not be related to the plugin itself?