Fade out not happening...

Problem/Motivation

Colorbox installed on a complete blanco Drupal install is not rendering fade out on a colorbox slide show.

Steps to reproduce

Drupal clean install 9.4.8
Colorbox 2.0.0
jquery.js v3.6.0 (part of Drupal core/assets/vendor/jquery.js)
jquery.colorbox.js 1.6.4
colorbox.settings

_core:
default_config_hash: GXgpFNb8H7n-27gImTB9s1A2lE-lmH43d6qf2KMYp_A
dompurify_hide_warning: false
custom:
style: plain
activate: 1
transition_type: fade
transition_speed: 350
opacity: 0.85
text_current: '{current} of {total}'
text_previous: '« Prev'
text_next: 'Next »'
text_close: Close
maxwidth: 98%
maxheight: 98%
initialwidth: '300'
initialheight: '250'
overlayclose: true
returnfocus: true
fixed: true
scrolling: true
slideshow:
slideshow: 1
auto: true
speed: 2500
text_start: 'start slideshow'
text_stop: 'stop slideshow'
advanced:
unique_token: 1
mobile_detect: 1
mobile_device_width: 480px
caption_trim: 0
caption_trim_length: 75
compression_type: minified

Is not linked to the browser; same problem firefox, chrome and safari

I tried to debug in the browser and it seems the fade out is just not happening; on time out (oncomplete) the parent container with the dissapearing image is just removed in jquery, which provokes the sudden flash. I suppose it is the white background that appears because the image and surrounding container cboxLoadedContent is removed...

I see no reason why the fade is not happening... perhaps a jquery version conflict???

Hope someone can help me out.
see two small screencaptures of slideshow and of the html while rendering...

Kind regards,
Erwin