You can skip the step. It is not essential to run at this time, it is just rebuilding automatic page descriptions. Do URL &step=NEXTSTEPNUMBER
Replace NEXTSTEPNUMBER with the actual number of the next step (it varies, but it's current-step-number + 1).
Development note: need to modify the parser so that it doesn't generate any views during the upgrade process, since XenForo Views are not compatible with streaming content (headers already sent).
I do find it odd that I have been able to run the upgrade script successfully on XenForo. But then I have rarely ever seen the headers already sent error in any variation -- it is possible that different PHP configurations treat the error more severely.