We also got this error when we upgraded from Hudson 1.372 to Jenkins 1.403 on a Ubuntu box. We had several Hudson servers that we upgraded, and only one had this issue.

Got this error when upgrading from 1.398 to 1.403

I think this is just a Jenkins 1.403 bug.

I upgraded from 1.398 to 1.403 and have this problem.

For anyone who is experiencing this problem, I have found a temporary workaround. First create a new job with the new Jenkins, then add a build step in the new job. After that, you should be able to add build steps to any existing jobs that wouldn't have worked before.

Using Chrome's developer tool or Firefox's Firebug extension, we get this error when adding a build step:
POST http://<HOST>/$stapler/bound/43328235-2ba7-416b-bda7-66b3834cc54c/render 404 (Not Found)

I had the same problem with the Jenkins ver. 1.408
but it occurred as not Jenkins but browser issue - it did not work in Firefox 3.6.15, but it works as normal in Opera 11.10, in Firefox 4.0, and in Chrome 10.0.648.205

I was having this issue on Ubuntu 10.10 and Jenkins 1.413. The problem was that "Prevent Cross Site Request Forgery exploits" was checked. After deactivating that, I no longer had the issue.

For ours, the "Prevent Cross Site Request Forgery exploits" was never checked.

I see the problem on Jenkins 1.421
"Prevent Cross Site Request Forgery Exploits" was never checked. The malfunctioniing "Add Buidl Step" button occurs on both IE 8.07600 and Firefox 3.6.18. The work around of creating a new job and adding a build step did not work for me as the "Add build step" remains broken in the new job.

Since this bug effectively disbales new developement and test functions, I bumped the priority to "blocker" as defined in Bug tracker bug entry screen.