Similar message is seen in JENKINS-64164 for the scm-api plugin. JENKINS-64164 may be a duplicate of this one.

sircmpwn could you please follow the instructions specified on the https://issues.jenkins.io/browse/JENKINS-64072 description to try find and out which plugins may have been causing this issue. 

Also, could you please try updating the installed plugins? Tens of plugins have been fixed since this issue was reported, maybe your use case can be fixed already.

I checked and installing ircbot on it's own is enough to reproduce this

After discussion in the IRC, I'm trying to solve the problem but know next to nothing about Jelly so it might take a few iterations (assuming the problem really is due to HTML tables in the markup - stacktrace did not convince me of that).

The first shot is here: https://github.com/jenkinsci/ircbot-plugin/pull/31

With the newly released ircbot-plugin 2.35 and instant-messaging-plugin 1.29 applied, now the ircbot post-build step are in appropriately fomatted "main panel" part of the legacy job config page.

I am not sure if the Add channels button works right, because it (and help bubbles) do not show anything visibly :\ (the help bubble for ircbot is expanding a div with text, one just below it with help for Channels is not - but maybe I just can't find them on-screen due to other plugins with broken markup messing up my deployment)

But the Strategy choicelist, Extra Message string, 5 notification type checkboxes and a Channel Notification Message choicelist - these are in the "correct-looking" part at least

Thank you! Looks good now.