Ensure that your MPLAB® 8.x project is saved from MPLAB® 8.14.03A or later.
Ensure that all files referenced by a #include directive are included in your project tree. The conversion utility may not add the paths to files not explicitly included in the MPLAB® 8 project.
Ensure that file names referenced by #include directives are spelled with the proper case (e.g. p24FJ128GA010.h - not p24fj128ga010.h). non-Windows® operating systems are case sensitive.
Ensure that relative paths used in #include directives use forward slashes '/' and not back slashes '\'. The forward slash is recognized on all supported operating systems while the backslash is only recognized on Windows®. Better still would be to define the paths in the project configuration.