Hi @arpinux,
Found it. Thanks for the logs and for testing both editors on a clean install before posting, that ruled out a config problem on your end immediately.
It's not EasyMDE or TUI Editor specifically, and it's not the theme either. Both editors only load their CSS/JS on pages that actually need them, to avoid shipping 200+ KB on pages that don't. That check runs against a list of known URL patterns, and the pattern for categorized discussions only covered the edit route:
/f/{category}/d/{id}/edit
not the plain show route:
/f/{category}/d/{id}
which is the page your reply form actually lives on. So on any install using categorized URLs, the discussion page never requested the editor's assets at all. No error in the console because nothing failed — nothing was ever asked for.
Fixed in both plugins by widening that pattern to match the show route too (EasyMDE 2.3.16 → 2.3.17, TUIEditor 1.3.13 → 1.3.14). Same one-line change in both, since they'd copied the same allow-list logic.
No need for admin access on your end, the code made the bug obvious once I knew where to look. This will be in the next release; I'll update this thread when it ships.
Update(plugins/EasyMDE/EasyMDEPlugin.php) or Update(plugins/TUIEditor/TUIEditorPlugin.php)
Search and replace :
'~^/f/[^/]+/d/[^/]+/edit~',
by
'~^/f/[^/]+/d/[^/]+~', // discussion catégorisée : show + edit + actions (formulaire de réponse)