I now know that mod_security2 is causing this problem. Disabling it is not a solution.
The module is blocking the POST:
Mon Sep 08 16:34:57 2008] [error] [client 213.22.159.193] ModSecurity: Access denied with code 400 (phase 2). Pattern match “\\\\%(?!$|\\\\W|[0-9a-fA-F]{2}|u[0-9a-fA-F]{4})” at ARGS:selection. [id “950107”] [msg “URL Encoding Abuse Attack Attempt”] [severity “WARNING”] [hostname “bugflux.org”] [uri “/wp-admin/options-permalink.php”] [unique_id “KVUMRH8AAAEAAGwKyAoAAAAI”]
I’d like to know if there’s any workaround or solution for this as my host is not willing to disable it, and this should not happen anyways, I believe…