Now the problem is that MobiPocket does not always work with IE9 and above. It often breaks when you create Tables of Contents.
I was creating Tables of Contents like this (which used to work just fine):
and receiving this error:
I scoured the web for fixes and I found some really complicated ones. However, Soph (my sister) was having no such issues and finally we were sitting together when she gave me a very significant piece of information. She puts 'p' or 'div' in the tag name box when she creates a table of contents, like this:
Low and behold, no error message. After editing HTML, trying to use the guide option rather than the table of contents tab and everything else, it was a simple as not leaving a field blank.