Settings of Filtered HTML input format won't save

What are the steps required to reproduce the bug?

1. Go to example.com/admin/settings/filters/1/configure
2. Change some settings (like changing the «Allowed HTML» tags in the «HTML filter» fieldset, or check the «Spam link detterent» check button.
3. Save

What behavior was I expecting?

I am expecting to see this message: "The configuration options have been saved."
I am also expecting that the settings are saved.

What happened instead?

No message, nothing changed, nothing saved.

MySQL-database 4.1.22, PHP 5.2.2, Apache/1.3.37 (Unix) mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 FrontPage/5.0.2.2635.SR1.2 mod_ssl/2.8.28 OpenSSL/0.9.7a PHP-CGI/0.1b

As much information as I can

Tested on 4 sites running Drupal 5.6, where 1 is fresh install and the 3 are upgraded from Drupal 5.5. All with the same bug. I posted a forum topic at Drupal Norge (ref: http://drupalnorge.no/innlegg/forum/innlegg-753) just 1 ¹/₂ hour ago on this, and I just received one reply at the moment (it's 6:30am here), reporting that she encountered this bug as well.