First have you tried creating a new print layout, not a duplicate as that will just copy any corruption there?
After that I am thinking perhaps the quickest troubleshooter in the long run would be divide and conquer.
Zip up the project so you have a copy that cannot accidentally be opened and worked on. That gives you a copy that you can safely revert to later if needed. Save the zip file somewhere safe.
Then create another copy of the project so that you have two copies that can be worked on.
Open Copy 1 and delete half the topics. Ignore any broken links reported. Generate the document from that copy.
Open Copy 2 and delete the other half of the topics. Generate the document from that copy.
It should be the case that one copy is now OK and one still has the problem.
Trash the good copy and then create a second copy of the copy with the problem.
Repeat the process until you have located the topic that is causing the problem.
The process takes less time than you might think.
See www.grainge.org for RoboHelp and Authoring tips
I'll give it a go and report. Thanks.
Well, after a lot of testing, it turned out that the problem was one line that somehow sneaked into my stylesheet:
After I removed this line, everything worked.
Yes, damn their eyes.
Thanks, as always for your suggestions.