Please navigate to Dashboard > wpDiscuz > Settings > Comment Content and Media admin page, make sure the post type is checked in the "Allow Media Uploading for Post Types" option. 

we took over a wordpress site of our client and there were those buttons in comment form to add photo & video to users comment. Suddenly they're gone. It's working fine on local copy of project.

I can see

class="wmu-add wmu-single"

in

#wc-main-form-wrapper-0_0 > form > div.wc-form-footer > div.wpdiscuz-textarea-foot > label:nth-child(2)

selector. But the one is missing on production. I'm wondering is this a paid plugin issue, because in the admin dashboard the Media Uploader section in Addons tab is all grayed out. Or are those buttons not connected to this plugin and are created by some separate settings? I went through all WPDiscuz options but none of those brought back the buttons on place.