Unhandled win32 exception occurred in RoboHTML.exe [3632]. after setting style for header in Master Page

Using RH2015  and MS Word 2010.

[Version of RoboHelp edited by Moderator. Please always use the correct version name as it helps people with the same problem down the line find a post that may help them. In this case it was referred to as RH15 so a search on RH15 would fail.]

Using Master pages, more or less from the RH15 EmployeeCare3 sample project.

I created a new project, imported about 15 pages from that project and modified them to be specific to my project (shared with a half dozen other writers who lack RH experience).

I noted that the header.footer had additional paragraph marks, which took up additional space in the printed document..

I saw that the paragraph styles for the header were undefined, so I changed the paragraph style to "header" and set the above/below space to 0.

This made the topic body content disappear completely -- and then I recived the title as an error:

Unhandled win32 exception occurred in RoboHTML.exe [3632]

Any ideas?