Maybe it’s just hard to find because it’s such an edge case. Or is it?
Here’s the scenario: you’re converting an existing WordPress site that uses Custom Post Types (with archive pages) to Multisite.
Suddenly, when you’ve switched the site, your CPT archive pages return a 404 error.
Check this: insert /blog into the URL, like so…
Old URL
http://example.com/my-cpt-archive
New URL
http://example.com/blog/my-cpt-archive
Does it work? If so, good. If not, I can’t help you. *shrug*
Let’s just assume it does work, and continue…
You see, Multisite inserts /blog into the URL to prevent URL conflicts between the different sites. Problem is, it’s kind of stupid about it, especially if your site is not a “blog” (and despite what the core team thinks, I’m pretty sure most WordPress sites these days are not blogs). It doesn’t do anything to change page URLs, which are just as likely to conflict.
Anyway, there are two things you need to do. First, go to Settings > Permalinks. (Note that /blog has appeared in all of the permalink structures!) Switch to “Default”, save, then switch back to whatever you want it to be and save again. (Note that /blog has disappeared!)
This still isn’t going to fix your CPT archives though. For that you need to go into your functions.php file in your theme, or wherever you are registering the CPTs in your theme/plugin. In the register_post_type() function, you may have 'rewrite' defined, like this:
'rewrite' => array('slug' => 'something'),
Change it to this:
'rewrite' => array('with_front' => false, 'slug' => 'something'),
You’ll need to flush the rewrite rules by temporarily adding flush_rewrite_rules(); in the functions.php file, uploading it, loading a page, and then removing the code and re-uploading the file. Or, you can refresh the Settings > Permalinks page. (Much easier, but I haven’t tested to be 100% sure it works in this case.)