Hi BaenSidhe,
Thank you for your replies and suggestions!
In the meantime, jackie_w has provided me with helpful instructions that fixed my problem (and some other knowledge that explain why he/she is a "Wizard" while I am a "Junior member"
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In my case, I needed to delete a (maybe obsolete) entry in the .opf file after having converted the book with Calibre (epub>epub). Strangely, the total of page numbers have been reduced a bit, but the content seems to be intact and without any page number malfunction or any forced jumps to a weird negative number.
I will keep your suggestion to use Sigil's validate epub function in mind!
Thanks!