Can’t access WordPress site in subfolder possibly because of .htaccess redirect from root installation?

Just comment out the line

RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]

or change to

RewriteRule ^([_0-9a-zA-Z-]+/)?(beta|wp-(content|admin|includes).*) $2 [L]

in the root .htaccess file.