Invalid File Path when exporting to OAM

Hi all,

Need a little help, my illustrator has just started using Animate and has made a series of animations (Animate documents). He has succesfully exported most of them as OAM files but one in particular stops at the first hurdle and displays "Invalid File Path".  We have looked through all of the setting and compared all of the files and there appears to be nothing in the preferences that is different across all the documents.  The files are all created and stored in the same folder.  As it is a new program to us both it is a bit like the blind leading the blind, so I thought I would ask the experts.