The mwext-VisualEditor-* jobs are no longer taking the submodule into account when starting a build.
It checks out the commit of the root repository, but the submodules are stale from whatever commit/branch was last used on that slave/workspace before the bug started happening.
While all VE jobs are affected, this is most significantly affecting mwext-VisualEditor-npm because that job asserts the git checkout is clean.
I temporarily fixed it by updating the live job config. It looks like the "Disable submodule" option no longer exists, and is now inside a "Advanced submodule options" menu. Since this sub-section did not exist in the XML, this part of the plugin was not triggered at all.
Looking at the diff you can see this is caused by the Git plugin having been upgraded and it has a breaking change.
<scm class="hudson.plugins.git.GitSCM" plugin="email@example.com"> .. <disableSubmodules>false</disableSubmodules> .. </scm>
<scm class="hudson.plugins.git.GitSCM" plugin="firstname.lastname@example.org"> .. <extensions> <hudson.plugins.git.extensions.impl.SubmoduleOption> <disableSubmodules>false</disableSubmodules> .. .. .. </scm>
jjb will need updating, and re-generating all relevant jobs.