plugin upload blank page

Hello,

i have i-MSCP 1.3.5 (httpd) and want to upload the phpswitcher plugin. But i get only a blank white page after clicking on upload on settings_plugins.php.

I also searched for error logs, but cant find one.

Other plugins like SpamAssassin (137kb) have the same problem.

Both is already empty.
access.log in /var/log/nginx say by this post request status 200, so there is no error in error.log.
i tried to reinstall interface multiple times, with and without ssl, also reconfigure, but still same result.

Is there another way to install plugins without the uploader?