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Put this code as well:

#cboxWrapper div#cboxContent {
      margin-top: 0 !important;
}

Could you please check on this pop-up? 

Thanky you!

https://ucimsenemecky.cz/dekuji-za-vstup-do-kurzu-audio_kopie/

Please see how it looks in my case: https://www.screencast.com/t/6mpnXzSWEzd

add this code as well:

#cboxWrapper div#cboxContent {
    height: auto !important;
}
#cboxWrapper #cboxContent #cboxLoadedContent {
    margin-bottom: 0px;
}

Wp discuz disabled my pop-up windows. They appeared, but it was impossible to fill in the fields (email, name).

So I was supposed to add following css code to my website (see below)

The popups work again after using the code, but they appear weirdly... see her Online kurz německého jazyka ZDARMA (ucimsenemecky.cz) and screenshot:

Thanks  a lot  :) 

#cboxWrapper #cboxContent #cboxTitle {
     position: static !important;
}

Please navigate to Dashboard > wpDiscuz > Forms admin page, edit the form, and just click on the "Update" button. Then delete all caches and check again.