Internal error encountered, Failed to generate eBook (Robohelp 11)

Hello

I'm new to Robohelp (I've been a long term user of Framemaker for print publications).

The Ebook generation from the Single Source Layout pod has stopped working and I get this message: Internal error encountered,  Failed to generate eBook (Robohelp 11 eBook preprocessor 11.0.0.179)

Compiling G:\Development\Course Development\CONE-online\Characteristics-of-light\Robohelp\Characteristics-of-light\Characteristics of light\!SSL!\eBook\!epub_tmp_folder_0\Characteristics_of_light.epub ...

Preparing to create eBook...

Clearing output folder...

Preparing files for eBook...

Copying files...

Updating files...

Finished preparing in 4 sec(s)

Initializing compiler...

Generating eBook output - eBook Handler (11.0.2.240)...

Transforming XHTML topics...

Preparing XHTML topic "Questions_about_light.xhtml"...

Preparing XHTML topic "The_Electromagnetic_Spectrum.xhtml"...

Preparing XHTML topic "Using_wave_properties/Interferometry.xhtml"...

Preparing XHTML topic "Using_wave_properties/Mach-Zehnder_modulators.xhtml"...

Preparing XHTML topic "Using_wave_properties/The-wave-properties-of-light.xhtml"...

Preparing XHTML topic "Wavelengths_and_Frequencies_in_optical_networks/ITU_bands.xhtml"...

Preparing XHTML topic "Wavelengths_and_Frequencies_in_optical_networks/Light_sources_and_wavelengths.xhtml"...

Preparing XHTML topic "Wavelengths_and_Frequencies_in_optical_networks/Wavelengths_and_Frequencies_in_fibre_optics.xhtml"...

Preparing Table of Content...

Generating EPUB 3 output...

EPUB 3 output generation failed.

Internal error encountered,  Failed to generate eBook.

Could anyone give me any advice on what to do to fix it? What have I done? I did try installing Adobe Dig Eds 4 and it seemed to stop working after that, but I've tried uninstalling that and it still does the same. I also tried re-installing TCS5. The responsive HTML5 output still works ok.

Thanks in advance,

Sarah