Issue with uploaded images in media gallery with wordpress multisite

I use wordpress multiste and run the subfolder version. Media gallery works fine with the “base installation”.
However, there are no thumbnails and images shown in the media gallery in the subfolder site.
I already read, that there could be an issue with my .htaccess, so here it is:

# BEGIN WordPress
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 %{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]
# END WordPress

I also put a /tmp folder in the root directory of the server, but this also didn’t resolve the issue.