Error when adding download to queue

I get this error message

Fatal error: Call to undefined function plugin_manager_category_form_submit() in /var/www/auweia.org/hannover-bebt-neu/sites/all/modules/plugin_manager/plugin_manager.admin.inc on line 79

when wanting to add a module for installation.

It worked the first time, then I always get to this error message.
Maybe it is important that I did not succeed to install a module before - even though entering the MD5 Checksum. I always got to: "Nothing has been selected to install."