yeah, but like I said, Organize Series doesn't do any 404 handling or redirects for taxonomy archive pages.
The ONLY rewrite rules and template_redirects that orgSeries does is for the seriesToc page (the table of contents page that lists all series) as there is no default WP route for something like that. But all taxonomy archive pages are left to WordPress to handle.
So it's either a theme thing, or another plugin that's causing the problem. But either way, you need to look at whatever is handling 404's for archive pages that DO exist but have content that is restricted.
What I can tell you is this tho.
1. http://stellamoega.com/series/sib/ is returning a 302 code to the browser (which redirects obviously to your custom 404 page)
2. http://stellaomega.com/series/seb/ is returning a 303 code.
Something you can do to verify this isn't an Organize Series problem. Create a category, but assign only restricted posts to it. Then go to the archive page for that category. I predict you'll see the same problem.