CKEditor upload (and pasted) files do not 'stick'

Problem/Motivation

We have suddenly run into a problem when uploading (including pasting) image files into the CKEditor.

Reproducing the error

When we attempt it, the files DO upload into the site filing system in the correct, expected location so the issue is not a server permissions error but...

The image appears momentarily in the (dimmed) CKEditor frame but then a pop-up message appears as shown below and after clicking to close the message, the inserted images disappears! ...

Can anyone please help/advise here as we are in big trouble if we cannot add images to our content areas :-(

I am not super hot on AJAX things but I have read enough to know that inspecting header is useful and it looks like this...

Request URL
https://bit-by-bit.org/ckeditor5/upload-image/full_html_limited_headings?token=GOpUnGF6a_ed2JOOY5Oy6OSwce5Vet0-gBH-iuOK80A
Request method
POST
Status code
201 Created
Remote address
91.238.164.175:443
Referrer policy
strict-origin-when-cross-origin
cache-control
must-revalidate, no-cache, private
content-language
en
content-length
130
content-type
application/json
date
Sat, 08 Nov 2025 06:37:25 GMT
expires
Sun, 19 Nov 1978 05:00:00 GMT
onsuccess
unset X-Content-Type-Options
server
LiteSpeed
x-content-type-options
nosniff
x-frame-options
SAMEORIGIN
x-generator
Drupal 10 (https://www.drupal.org)
x-memory-limit-memory
256M
x-memory-limit-override
0
x-powered-by
PHP/8.4.14
0
text/javascript
:authority
bit-by-bit.org
:method
POST
:path
/ckeditor5/upload-image/full_html_limited_headings?token=GOpUnGF6a_ed2JOOY5Oy6OSwce5Vet0-gBH-iuOK80A
:scheme
https
accept
application/json
accept-encoding
gzip, deflate, br, zstd
accept-language
en-GB,en;q=0.9
cache-control
no-cache
content-length
10120
content-type
multipart/form-data; boundary=----WebKitFormBoundaryTVwE8Mpko3fKzXfa
cookie
SSESScc6dee69d87c43f00313fee0f169c3ce=z03XnGfbM2vSmIg-np3EpwH2s8xYvNHiFEVvg7Bg244njRxa
origin
https://bit-by-bit.org
pragma
no-cache
priority
u=1, i
referer
https://bit-by-bit.org/node/8609/edit?destination=/node/8609
sec-ch-ua
"Chromium";v="142", "Google Chrome";v="142", "Not_A Brand";v="99"
sec-ch-ua-mobile
?0
sec-ch-ua-platform
"Windows"
sec-fetch-dest
empty
sec-fetch-mode
cors
sec-fetch-site
same-origin
user-agent
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/142.0.0.0 Safari/537.36

System

Drupal Version: 10.5.5
Web Server: LiteSpeed
PHP 8.4.14
Memory limit: 512M
Database
MariaDB 10.6.23-MariaDB-cll-lve

Help!