if you have started by using a demo installation. F.e. https://kriesi.at/themes/enfold-one-page-portfolio/
This homepage behaves as you described.

This option of header you can choose to have on the page editor. Have a look to the layout and find: “Header visibility and transparency”
That dropdown :

Make your selection there.

or without uploading – just with base 64 data uri:

#av-cookie-consent-badge:before {
  display: block;
  position: relative;
  content: "";
  width: 100%;
  height: 100%;
  top: 10px;
  left: 10px;
  background-image: url(data:image/svg+xml;base64,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);
  background-size: 30px;
  background-repeat: no-repeat;
}

by the way – here is a huge amount of usefull path – with the option to get the base 64 data uri:
https://www.svgviewer.dev/svg-to-data-uri

Enter f.e. cookie on search input field.

As a participant i could not see private content.

why do you think that there is something wrong?
is on your dropdown list no custom svg ?

you know how to have a child-theme replacement script?

you have found the solution on the element itself? – because i see images on your tabs.

is this a transparent header on desktop view?
then you had to adjust on fixed non-transparent header the padding-top of #main

but for your trials to have sticky header on mobile – my advice is to erases the code and replace it with ( It is difficult to identify the issue with the existing code. Therefore, I would prefer to start over.) :

@media only screen and (max-width: 1023px) {
    #header {
        position: fixed !important;
        height: 120px !important;
        max-height: 120px !important;
    }

    #top #header.av_header_transparency #header_meta {
        background-color: transparent;
    }

    #header_main {
        border-bottom: none;
    }

    #header:not(.av_header_transparency) #header_main {
        box-shadow: 0 5px 5px rgba(182,182,182,0.75);
    }

    .responsive #top .av-logo-container ,
    .responsive #top .logo a,
    .responsive #top .logo img,
    .responsive #top .logo svg {
        height: 120px !important;
        max-height: 120px !important;
        line-height: 120px !important;
    }

    .responsive #top .av-main-nav .menu-item-avia-special a {
        height: 120px !important;
        line-height: 120px !important;
    }  

    .responsive.html_mobile_menu_tablet #top #wrap_all .av_header_transparency {
        background-color: transparent !important;
    }

    #top .header_bg {
        background-color: transparent !important;
    }

    #top #header:not(.av_header_transparency) .header_bg {
        background-color: #FFF !important;
    }

    .responsive.html_mobile_menu_tablet #top .av_header_transparency .logo img.alternate, 
    .responsive.html_mobile_menu_tablet #top .av_header_transparency .logo .subtext.avia-svg-logo-sub {
    display: block !important;
    }

    .responsive.html_mobile_menu_tablet #top .av_header_transparency.av_alternate_logo_active .logo a > img, 
    .responsive.html_mobile_menu_tablet #top .av_header_transparency.av_alternate_logo_active .logo a > svg {
    opacity: 0;
    }

    /*** das hier individuell anpassen je nach dem  ***/
    #top #header.av_header_transparency #header_meta .phone-info * {
        color: #FFF !important;
    }

    .html_mobile_menu_tablet .header_color  #header.av_header_transparency div .av-hamburger-inner, 
    .html_mobile_menu_tablet .header_color  #header.av_header_transparency div .av-hamburger-inner::before, 
    .html_mobile_menu_tablet .header_color  #header.av_header_transparency div .av-hamburger-inner::after {
        background-color: #FFF;
    }
    .html_mobile_menu_tablet .header_color  #header.av_header_transparency .menu-item-search a:before {
        color: #FFF;
    }
    .responsive.html_header_top.html_mobile_menu_tablet  #top #main {
        padding-top: 120px !important;
    }
    .responsive.html_header_top.html_header_transparency.html_mobile_menu_tablet  #top #main {
        padding-top: 0 !important;
    }
}

and next: synchronise the switch point of your burger menu with those settings for your header.
means

@media only screen and (max-width:1023px) {
  #avia-menu .menu-item {
    display:none
  }
  .av-burger-menu-main.menu-item-avia-special {
    display:block
  }
}

for your content inside the first section there should be more distance to header on smaller screens! …
in addition :

@media only screen and (max-width:767px) {
  .responsive.html_header_top.html_header_transparency.html_mobile_menu_tablet  #top #main #av_section_1 .content {
    padding-top: 150px
  }
}

and what if you switch temporarly to a standard theme. Do the wp-admin buttons show the same problem?

do you have only standard entries in your htaccess file – or are there additional custom rules inside?

yes – and it is under Tools->Site Health->Info->Database

And you have choosen that setting on : General Layout – Content Alignment : “Center Content”

it is just below that option where you have choosen the left sidebar header

can you mail me the link too – if you do not like to make it public

do you use WPML on that page?

see for example: https://kriesi.at/support/topic/header-structure/
and there is a link to e.g. synchronise the languages https://kriesi.at/support/topic/implementing-the-same-theme-setting-to-all-languages-using-enfoldwpml/

I think this symbol appears when an image or file cannot be found.

Edit : now i have inspected your page with dev tools – my pages with full-width easyslider and video inside as list item – looks totaly different to your DOM.

I got this <div id="mep_0" … container inside <div data-rel="slideshow-1" class="avia-slide-wrap " ….with all the video instructions inside

Ah ok : this seems to be a matter if the inserted url is a relative url and not an absolute url.
by the way it seem to be the only way to show the video on iPhones by relative urls – but than a message is shown : av-fallback-message if there is no fallback image.

so try: let there be the relative url to your video – do not enter a fallback image and:

#top .av-fallback-message,
#top .av-fallback-message + img {
  display: none !important;
}

or if you do not like to use the Sibling Combinator :

#top .av-fallback-message {
  display: none !important;
}
.avia-slideshow li img[src*="undefined"] {
  display: none !important;
}

Check what page-id is it where you do not like to show it – and then :

#top.page-id-12345 .my_title_container {
  display: none;
}

However, if you only want this feature on all pages but not on archive or post pages, for example, you can modify the snippet accordingly.