I was also having this problem.
When you drag the slider to the second chapter (first chapter at 00:00:00:00), click once on the Add Chapter button.
Move the pointer slightly to the right of the chapter mark.
If you get a pop-up hint labelled “Position”, you need to move the pointer a little further away until you get a pop-up hint labelled “Chapters”. If you see a message “Chapter X”, where X is a chapter number, you need to move the pointer a little further until it reads “Chapters”.
You may need to lightly click on the chapter mark to turn it from blue to a green colour.
Grab the slider near the middle and drag it to the right. A yellow/orange bar will separate from the chapter marker, leaving the blue one in place. Drag this yellow/orange bar to the next chapter position. You will find that the “Add Chapter Button” is no longer greyed out and you can add a new chapter.
I’ll admit it is a bit of a pain but when you get the hang of it, it speeds things up.
Hope this helps.