First, MobileRead doesn't want NSFW links. Please use PM or
github when reporting NSFW sites -- or obfuscate the URL by only giving part, ie "/stories/user/VitalGroves/61718/Painting-Red-Room-White-Black-Widow on hentai-foundry".
I've edited your post for you this time.
Second, both referenced stories works fine for me.
I would need to see a debug log (Calibre Preferences dropdown, Restart in debug mode, attempt download) to know any more.