When a job configured using the build-flow plugin fails it is not possible to claim it. When you click the claim button a HTTP post is submitted with the following data:
reason=&sticky=on&Submit=Claim

Claiming a regular job results in something like this in the HTTP request
reason=&sticky=on&json=%7B%22reason%22%3A+%22%22%2C+%22sticky%22%3A+true%7D&Submit=Claim

Because of the malformed data in the submission you get an Error 400 with the message "Exception: This page expects a form submission"