If you are experiencing 404 errors with pretty permalinks and a white screen when you upload images, mod_rewrite may not be enabled in Apache by default. Mod_rewrite is an extension module of the Apache web server software which allows for “rewriting” of URLs on-the-fly. It’s what you need to make pretty permalinks work.
WordPress Multisite networks usually experience this but it can also occur on shared hosting providers or after a site migration or server move.
Reset your permalinks through Settings > Permalinks. If this does not work, you may have to edit the .htaccess file manually.
If you are not familiar with editing your .htaccess file, contact your hosting provider to ask them to turn on mod_rewrite rules.