When hovering over a queued item to check on the cause of blockage, the UI displays an extremely large text area showing something like the following:
The text contains every single node in the Jenkins instance. And on some systems completely fills the browser window obscuring all other text.
But it is intermittent and on the next refresh (probably by the automatic queue maintenance), the cause of blockage shows more simple cause like "Waiting for the next available executor for <label1>" (see simple-cause.png ).
(For that reason I have not been able to capture a screenshot yet. As soon as I can I will attach it to this ticket)
It sometimes appears that build parameters shows up in that text.
This seems related to the improvement made in https://issues.jenkins-ci.org/browse/JENKINS-38514.