Book which generates erroneous kepubs

[Dinth]

Hi. Ive been using kepubify for a long time now and never had any problems until now. Ive got a book (legit one, purchased) in epub format, which opens perfectly fine in epub, but when i convert it (without errors) it produces an errorneous kepub - on the Kobo reader im getting no content in the book, while when im trying to open it in Xreader im getting the following:

This page contains the following errors:

error on line 1 at column 141: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

As the book is watermarket, im happy to provide it to the dev via email.