I edited this thread because I already found the solution on my own. For anyone who is interested in the same thing, go to "components/com_easydiscuss/views/post/view.ajax.php in line 979

change this:

if( empty( $post[ 'dc_reply_content' ] ) )

to this:

if( strlen( $post[ 'dc_reply_content' ] ) < 120 )

In this case the minimum characters per answer is 120.