selfhosted background video does not play

Hi
the video (selfhosted and as background does not play)

– Its on the server
– the URL is correct
– it plays when you call it via URL directly
– Firefox latest

This is the code:

[av_section min_height='100' min_height_px='700px' padding='large' shadow='no-shadow' bottom_border='no-border-styling' bottom_border_diagonal_color='#333333' bottom_border_diagonal_direction='' bottom_border_style='' scroll_down='aviaTBscroll_down' custom_arrow_bg='' id='vidback' color='main_color' background='bg_color' custom_bg='' background_gradient_color1='' background_gradient_color2='' background_gradient_direction='vertical' src='' attachment='' attachment_size='' attach='parallax' position='top left' repeat='stretch' video='https://domain.......de/video/film.mp4' video_ratio='' video_mobile_disabled='aviaTBvideo_mobile_disabled' overlay_enable='aviaTBoverlay_enable' overlay_opacity='0.2' overlay_color='#000000' overlay_pattern='' overlay_custom_pattern='' av_element_hidden_in_editor='0' av_uid='av-auo1aw' custom_class='vidbackclass']
[av_heading heading='I am a <strong>heading</strong> blablabla' tag='h2' style='blockquote modern-quote modern-centered' size='50' subheading_active='subheading_below' subheading_size='18' margin='' margin_sync='true' padding='35' color='custom-color-heading' custom_font='#dbdbdb' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' av_uid='av-269oh4' custom_class='' admin_preview_bg='']
Lorem ipsum dolor
[/av_heading]
[/av_section]

The domain has made invisible by purpose for this thread.
Claudia