Without the www isn’t reachable, with the www is. The redirect has no www therefore the redirect will not work as the address redirecting (or in this case proxying) too is not reachable.
I would argue that a www on a subdomain (wiki in this case) is not a this usually done (though it is personal preference I guess) as the www is a subdomain of a subdomain.
As I mentioned previously, because you are proxying to the subdomain wiki.asellus.org, from asellus.org/wiki the wildcard/splat redirect would likely cause an infinite loop and fail (which it is.)
Try removing the wildcard/splat from the redirect in the wiki repository and the main site proxy should work again.
Of course, a wildcard/splat will match anything, including /, so it will still cause a infinite redirect loop
So when https://asellus.org/wiki/ is requested it is actually calling https://wiki.asellus.org/, but this path is redirected to https://asellus.org/wiki/ thus the loop begins
so leaving the domain level redirect aside for the moment, I encountered a new problem with the redirect when switching from jekyll to a legacy gitbook / honkit solution: the page builds correctly and is reachable under wiki.asellus.org, but the redirect fails now: the content is there, but the css is broken, and the intra-page links don’t work (the “wiki” is missing in the url).
In jekyll, I solved this with the baseurl option in the config file, but I am not sure there is a similar option for honkit. Do you have any suggestions how this could be solved from the netlify end?
Searching the git repository for the project, I do see basePath which is possibly a way to prefix all URLs with /wiki/ so they work under asellus.org/wiki/.
The simplest solution is to use absolute URLs for all your assets. Or store your assets in a different folder and you can redirect all those assets in one go, but this would probably be difficult to achieve.
It would have been possible to exclude a URL from the wildcard but that’s not possible in the proxy setup that you currently have.