So far, I made some projects in which I merged another CHM. This worked perfectly until now.
I merged the same CHM in another project but the result is not the same.
The TOC of the merged help appears in the parent TOC but when I click a topic in the TOC, nothing happens. Double-clicking also does nothing. When I search for text from the merged CHM, then I find it and I can even click the result which shows the merged help topic. Also clicking a link in the merged help topic to another topic of the merged help works fine. When I then view the TOC and click Back and Forward, the merged help topic is selected in the TOC. However, clicking one of the already viewed merged help topics still doesn't show the topic.
Up to now, I did not yet find any setting that is different between the good and bad projects.
Hopefully someone if familiar with this problem and knows the solution.
It seems that child CHMs should not contain underscores. I tested and it works now for the problem project.
There however is still must be a reason why underscores in the child CHM worked well with my other projects.