[KevinH]

The unique id is used to encode/decode embedded fonts. Two different font encryption approaches exist (one Adobe's and one Idpf) and exactly how they encrypt/decrypt and exactly what part of the unique id string they use also seems to be not well specified so there are variations on that as well.

For Adobe's version I would remove the "urn:uuid:" prefix from the unique id in the content.opf and the toc.ncx (making sure they match) as I think only the uuid part is used. For idpf, they use whatever is provided in that unique id , leading to font encryption vs decryption issues depending on how these routines were implemented across devices.

Hope something here helps.

KevinH