Today, I wasted major part of my day trying to figure out why my subfolder based WordPress multisite install is redirecting all 404 errors to the site’s home page.
The problem turned out to be the constant “NOBLOGREDIRECT” which I defined in wp-config.php file. This constant ensures that in a multisite install, if anyone tries to access a non-existent site, they will be redirected to the signup page.
Now, this works great in case of subdomain based WordPress multisite setup. But, for subfolder based multisite setup, it actually starts redirecting all 404 pages to the signup page.
I recently converted my WordPress network from subdomain to subfolder structure and that’s why this setting started creating problem after that. If you have a subfolder based multisite install, you do not need NOBLOGREDIRECT.
Hope that helps.