Multi site issue

In network admin, go to Tools > Network Setup and verify the setup code provided is actually in place. For the .htaccess part, be sure there is no other WP rewrite code besides what is shown in network setup. Old single site code still in place will cause the issues you’re having.

Sorry about “Tools”, network setup has been moved to “Settings”. Your setup looks OK AFAICT, but please double check with the setup configuration shown for your site.

I think the Litespeed cache could be interfering. As a test, try deactivating it. Also clear your browser’s cache before trying the subsite URL again.

In network admin, go to Settings > Network Setup. It’ll show what the contents of wp-config.php and .htaccess should be. Compare with your actual file content. Correct fie content if necessary.

Also check you permalink settings. Loading the screen causes the rewrite rules to be regenerated, which often resolves issues like this.

If that doesn’t help, the current theme or one of your plugins could be interfering. To test this theory, switch to a twenty* default theme and deactivate all plugins. Visit permalink settings yet again before testing your sub-site URLs.

If flushing rewrite rules and switching themes and deactivating plugins as I suggested earlier do not help any, the only other thing I can suggest is doing a manual update. You can try just using the re-install button on the updates screen, but since we’re trying to correct faulty code, this may not do the job. Manually replacing the files is more reliable IMO.