htaccess – Only plain permalinks working

I’ve a problem with a site (part of a multisite structure).

All of a sudden all the static pages are showing the list with the last posts, except for the homepage. The single posts are working fine.

If I change the permalinks from “post name” to “plain” everything is working fine.
here the htaccess file.

RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - (L)

# add a trailing slash to /wp-admin
RewriteRule ^((_0-9a-zA-Z-)+/)?wp-admin$ $1wp-admin/ (R=301,L)

RewriteCond %{ENV:REDIRECT_STATUS} 200 (OR)
RewriteCond %{REQUEST_FILENAME} -f (OR)
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - (L)
RewriteRule ^((_0-9a-zA-Z-)+/)?(wp-(content|admin|includes).*) $2 (L)
RewriteRule ^((_0-9a-zA-Z-)+/)?(.*.php)$ $2 (L)
RewriteRule . index.php (L)

what could it be? Any hint?
thank you