The project was shared and I have found a topic causing the problem, there may be more!
I am not clear how it was not found using the method I suggested, gradually adding more and more to a new TOC as that is how I found it. I created a new TOC and dragged a bunch of topics from the Contents panel to the TOC. In the first instance all the topics beginning with A. No folders as the sole objective of this TOC is troubleshooting.
It compiled just fine. So then I add the Bs and it still compiled. I then added the Cs and it failed. So then I deleted all the Cs and added them back in a few at a time. Eventually I found Custom Job API was what was causing it to hang. Looking at the code of the topic reveals it has either been imported or pasted in from Word at some point as there are many references to MSO styles. They don't exist in the CSS so RoboHelp doesn't know what to do and hangs.
I selected all the content and clicked the Clear Formatting icon and it then generated OK. Obviously the topic will need a bit of formatting work.
What you need to do now is create a TOC and add all the topics down to but not including that file. It should generate without issue. Then clean up that topic as above and add that. Generate again to prove it works for you. Then keep adding 20/30 topics at a time and generate again until it fails. When and if it does, then you know you have another topic needing clean up.
There are more. After fixing that in the TOC I created that now works, I reverted to yours and it still doesn't work.
Use the menu (bottom right) to mark the Best Answer or Highlight particularly useful replies. Found the answer elsewhere? Share it here.