Had not received this error prior to upgrading to 3.0.1. Compiling for Kindle KF8/Mobi Ebook.
Amazon kindlegen(MAC OSX) V2.9 build 1028-0897292
A command line e-book compiler
Copyright Amazon.com and its Affiliates 2014
Info(prcgen):I1047: Added metadata dc:Title “Title”
Info(prcgen):I1047: Added metadata dc:Creator “Author”
Info(prcgen):I1002: Parsing files 0000030
Info(prcgen):I1016: Building enhanced PRC file
Info(prcgen):I1015: Building PRC file
Info(prcgen):I1006: Resolving hyperlinks
Info(prcgen):I1008: Resolving start reading location
Warning(prcgen):W30013: Invalid “nav” element in Navigation file. Please refer to Kindle Publishing Guidelines about usage of EPUB3 Navigation support. A “nav” element can have two child elements, an optional heading and a mandatory ol ordered list in file: /private/var/folders/fp/r0snfbhj5099vmyqlz979bbw0000gn/T/EBOOK_TEMP/Book/contents.xhtml line: 8
Error(prcgen):E24012: Epub Navigation file could not be built.
Info(prcgen):I1007: Resolving mediaidlinks
Info(prcgen):I1011: Writing mediaidlinks
Info(prcgen):I1009: Resolving guide items
Info(prcgen):I1038: MOBI file could not be generated because of errors!