Hi Liesje,
May be that you found a solution, but I found this forum, and your question, yesterday while searching for an answer to the same problem.
I was able to split the audiobook file (14+ hours, nearly 500MB) into two mp3 files by using the free audio editing program Audacity.
When I opened the resulting files in Slice, each showed a positive number for "length."
Slice then had no trouble splitting them into as many tracks as I wished.
I hope that it works for you as well.