I want to thank all of you guys for your help!
I try the suggestions you gave me and I think (accidentally) find a solution for that.
Converting my file to .htm and then to .epub (via Calibre) did the trick and the file opens without errors in Kobo
IF the epub sideloaded on the device directly as it is.
When I drop the file into Sigil for a few adjustments (such as generate TOC and make some chapters) the saved file won't load in Kobo and send me a false DRM error. Something's strange with the structure of the file? The epub itself have a lot of sections (about 65) labeled as: Agkhone2_split_000.htm, Agkhone2_split_001.htm e.t.c.
My final change is to rename the sections as: Section001.htm, Section002.htm e.t.c and voila! (the sun shines again!)
The sideloaded epub works fine on my device!
I don't know what wrong is happened... Something with the underscores or the long section names?
Anyway, I can now read my book at nickel... Finally!
Thank you all, again!