I took a look at the calibre source code to see what could cause that error. I found that it will happen if the text color is set to a decimal value, which is not allowed for a color. For example this will trigger that error:
Code:
<p style="color:1.5">Lorem ipsum dolor sit amet</p>
This will fail to convert to MOBI even with the current calibre version.
You should be able to find the problem in the original book file by opening the EPUB in the calibre editor and using the "Check book" tool.