Genau die gleiche / ähnliche Meldung bei mir.

Warning: Array to string conversion in … …/wp-includes/script-loader.php on line 2733

Fatal error: Uncaught TypeError: explode(): Argument #2 ($string) must be of type string, array given in 
… … /wp-includes/class.wp-dependencies.php:166 Stack trace: #0 
… … /wp-includes/class.wp-dependencies.php(166): explode('?', Array) #1 
… … /wp-includes/class.wp-styles.php(371): WP_Dependencies->all_deps(Array, false, false) #2 
… …/wp-includes/class.wp-dependencies.php(106): WP_Styles->all_deps(Array) #3 
… …/wp-includes/script-loader.php(2739): WP_Dependencies->do_items(Array) #4 
… …/wp-includes/class-wp-hook.php(303): wp_add_iframed_editor_assets_html('') #5 
… …/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array) #6 
… …/wp-includes/plugin.php(470): WP_Hook->do_action(Array) #7 
… …/wp-admin/admin-footer.php(105): do_action('admin_footer-po...') #8 
… …/wp-admin/post.php(369): require_once('/www/htdocs/w01...') #9 {main} thrown in 
… …/wp-includes/class.wp-dependencies.php on line 166

if i do deactivate the implemented layerslider the error message is gone

well youtube and vimeo do offer now little episodes of a mp4 film as gif to use it for preview films.
but i know that the offered gifs do not always play as it should.
I have used the converters here: https://ezgif.com/video-to-gif

But for the usage as “featured image” they had to fullfill some conditions. Main is size – or frame-rate. – and do not insert a recalculated ( resized image) to use as featured image. so resize it on the link above yourself to get a nice dimension to use.

Maybe this article will help you to solve your problems: https://betterstudio.com/blog/gif-featured-image-wordpress/#how-to-fix-gif-not-animating-in-wordpress

You are welcome

Hi Günter – thanks

Yes, that is definitely the better phrase.
you see the word by word translation on German is misleading:
It is the oposite way as described.

This way with page array

if( is_page ( array ( 21, 8675, 13792, 14614 ))){ 

first : i do not like the big white rectangle when the letters change – then maybe a text-shadow will do the job:

#top .av-rotator-container-inner {
	text-shadow: 3px 3px 10px #000 !important;
}

.av-marked-text .av-rotator-text-single {
	background-color: transparent !important
}

or in additon a little outline with a modern method to add outline ( only IE11 does not support this) – replace the first rule with:

#top .av-rotator-container-inner{
	text-shadow: 2px 3px 4px #000 !important;
	-webkit-text-stroke: 1px black
}

if you like it with more browser support – that little trick will do – replace the first rule with:

#top .av-rotator-container-inner {
	text-shadow: 3px 3px 5px #000, -2px -2px 0 #000,   2px -2px 0 #000, -2px 2px 0 #000, 2px 2px 0 #000;
}

First of all – do you still need the jQuery Migrate on your installation?
It is still active. You can avoid loading it on Performance : “Disable jQuery Migrate”

This has nothing to do with your problem at first, but if your plugins are up to date, there should be no reason not to disable it.
If you experience problems afterwards, you can undo it.

__________

Do you use Gutenberg on your Enfold Installation? – or the Classic Editor?
Some Addons on Gutenberg Editor can cause such a problem ( f.e.: Ultimate Addons for Gutenberg )

you don’t have to insert the css code above – because it is part of that css belongs to layerslider – but on some reasons this is not loaded.
If you put in the css code above – you will see that the gray rectangle is gone.
But i do not see any of this .ls-hidden rules in your css.

maybe this is an alternative:
https://wordpress.org/plugins/menu-items-visibility-control/
there are additonal fields on each Menu-Item – you can insert conditional tags there ( f.e. is_home() or ! is_home() etc pp )
https://codex.wordpress.org/Conditional_Tags
so it will be possible to even have is_user_logged_in() etc.

the css rule for ls-hidden is missing in your stylesheet !
if you add to quck css this – you will see that it will work:

.ls-hidden {
    width: 0px !important;
    height: 0px !important;
    position: absolute;
    left: -5000px;
    top: -5000px;
    z-index: -1;
    overflow: hidden !important;
    visibility: hidden !important;
    display: block !important;
}

what is your setting on Enfold (Child) : Layout Builder :

• Integrated (Bundled) LayerSlider Plugin and
• Layerslider Options

i have no problems with layerslider this way.
Do you have installed a stand-alone Version? and do not use the implemented function?

Can you check your advanced Layerslider insertion on that page.
F.e. remove it and place again your layerslider – and save the page.