You could try clearing the output folder, and the folder mentioned in the error (the error one I think is supposed to be automatically deleted at the end of the generation process, but I could be wrong about that). Do it for both outputs to make sure everything's clear.
Also, you could try restarting the computer to make sure all applications and services are reset, then try generating the problem output. It's possible that a Word service is still running, and RH gets upset because it can't have two going at the same time. (also, I've had problems with outlook incorrectly holding on to a file reference and causing things to fail in older versions, for example.)
If those don't fix the issue, the divide and conquer approach seems like the way to go. First, make sure "Include all referenced topics" is deselected in the problem output and try to generate. This rules out problems in some topic that is not included in your TOC, but which is pulled in from a link in one of the topics.
If you still get the error, create two copies of your problem toc. Delete half the topics from one, and the other half from the second. Generate each to see if one generates and one fails. (Hopefully one works and one fails.) Repeat the delete/generate steps with the failed half until you can determine the topic causing the issue.
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