Thank you both for your answers.
I was mistaken by the file name and thought it was meant to convert Mobi dictionaries to the DICT format, my bad.
The dictionnaries I tried to extract where mostly successfully extracted despite the errors in the logs :
Error: Dictionary contains multiple inflection index sections, which is not yet supported
Error: Dictionary uses obsolete inflection rule scheme which is not yet supported
I'm not quite sure about what they really mean (if something will be messed up in the extracted HTML file or not) but the output HTML file is not empty and seems ok.