Welcome to the forum.
Does this problem also occur if you publish to an entirely different location? If it does then can you link the problem to any recent changes. If not, it seems to be time for the old divide and conquer approach.
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 help from that copy.
Open Copy 2 and delete the other half of the topics. Generate the help 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.
Let us know how you get on.
See www.grainge.org for RoboHelp and Authoring tips
I took a look at the last new topic I had created and noticed that some of the table formatting had gotten corrupted. Once I fixed the table, the error no longer occurred when I generated it.
Thanks for your suggestion!