When pasting/inserting an image in the client reply richtext editor (observed on /osta/old/ paths), the draft flow generates an invalid request to:

/osta/old/ajax.php/draft/ticket.client.<id>/index.php

This request returns 400 URL not supported.
After the failure, the editor inserts an  element with src equal to the localized error text (URL-adresse understøttes ikke) instead of a valid image URL, which triggers a secondary 404 request to:

/osta/old/URL-adresse%20underst%C3%B8ttes%20ikke

Technical review indicates:

Draft URLs are initially built as ajax.php/draft/<namespace> (and /attach) in js/redactor-osticket.js.
Draft routes in ajax.php do not support an extra /index.php suffix.
Dispatcher fallback for unmatched routes is 400 URL not supported, and the Danish translation matches the observed inserted text exactly.
Please investigate the draft/image paste upload flow and its redirect/error handling to determine why requests are being resolved to .../ajax.php/draft/.../index.php and why the non-JSON error response is being consumed as image source data.