Editor-button category picker broken in 2.5.1 - galleries.getCategories POST returns 403 Invalid Token

ENVIRONMENT
- Balbooa Joomla Gallery: 2.5.1
- Joomla: 6.1.2
- PHP: 8.4.23
- Browser: reproduced in a clean browser profile with no extensions/ad-blockers, so it is not a client-side blocker

SUMMARY
When inserting a gallery into an article via the Gallery editor button, the modal lists my galleries correctly. But clicking "All" in the Category column no longer does anything - previously it expanded to list the categories in that gallery so I could pick one. Nothing happens now, so I can't select a category through the UI at all.

STEPS TO REPRODUCE
1. Edit any article and click the Gallery editor button.
2. The gallery list loads fine.
3. Click "All" in the Category column for a gallery.
4. Nothing happens (the category list never appears).

EXPECTED VS ACTUAL
- Expected: the category chooser dialog opens, listing the gallery's categories to select from.
- Actual: no dialog appears; the request fails silently.

CONSOLE / NETWORK EVIDENCE
Clicking "All" fires this request, which is rejected:

POST /administrator/index.php?option=com_gallery&task=galleries.getCategories&id=1
-> 403 (Forbidden)

The server returns Joomla's core HTML error page (title "Error: 403", body "Invalid Token", body tag class contains "com_gallery ... task-getCategories"), and the modal's JavaScript then throws while trying to parse that HTML as JSON:

Uncaught (in promise) SyntaxError: Unexpected token '

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