My host resolved the issue (but don’t explain why it worked before). Here his answer :
“concernant Apache :
[Mon Dec 22 12:18:38.011393 2025] [:error] [pid 640924:tid 139886478915328] [client 88.125.5.26:5862] [client 88.125.5.26] ModSecurity: Access denied with code 44 (phase 2). Match of “eq 0” against “MULTIPART_UNMATCHED_BOUNDARY” required. [file “/etc/httpd/conf.d/mod_security.conf”] [line “30”] [id “200003”] [msg “Multipart parser detected a possible unmatched boundary.”] [hostname “fncta-midipy.fr”] [uri “/wp-admin/async-upload.php”] [unique_id “aUkpDZrOMI7eBWct1SazYAAA1Rs”], referer: https://fncta-midipy.fr/wp-admin/upload.php
c’est une erreur fréquente avec WordPress, j’ai assouppli une configuration sur le serveur. Pouvez-vous retester ?”
And it worked.
Anyway, thank you so much @scrambler (and @user) for trying to help me even it was not theme related. You are great !