I hear from JNC that their e-pubs pass EPubCheck so you might want to make a report with FB Reader. According to https://forums.j-novel.club/topic/6808/getting-encrypted-pages-on-a-premium-purchased-epub/6?_=1685790630062 the FB Reader Android app has had problems reading JNC novels, possibly due to JNC's E-Pubs now being uncompressed to improve performance. If that is the issue with FB Reader then the problem seems to be on FB's end as it appears that uncompressed EPubs are standards compliant. You might also be able to use some sort of tool or utility to repack the EPub using Deflate which might work, but it's not something I have ever looked into. I personally use Moon+ Reader Pro and although I haven't tried it with Wortenia generally JNC novels work fine.