[rickhan]

I usually see it in rar files with html content. The opf file has a bad spine reference. You can read them just fine, but they won't convert.
You can delete the reference, or just delete the whole OPF file. Calibre generates a new one.

This cause has nothing to do with deDREM software. I would think just ignoring this error would fix it, or regenerating the OPF file.