With trying with DOM inspector in firefox I found out that changing the action attribute of the form element solves the problem: remove "&action=foo". I don't know why, but it works.

before: "path/to/forum/profile.php?section=admin&id=35&action=foo"
after: "path/to/forum/profile.php?section=admin&id=35"

implementation in Greasemonkey:

// ==UserScript==
// @name           solve 406 error on punbb
// @namespace      personal
// @description    deletes "action=foo"
// @include        path/to/forum/profile.php?section=admin*
// ==/UserScript==

(function() {
    //remove "&action=foo"
    var form = document.getElementById('profile7');
    if (form){
        form.action = form.action.replace("&action=foo", "");
    }
})();

Nested color tags get only partially converted.

[color=#FF0000]this is red text [color=#0000FF]this is blue text[/color] this is the remaining red text[/color]

produces:
this is red text [color=#0000FF]this is blue text this is the remaining red text[/color]

a workaround is to use more tags, but the first version should work too.

[color=#FF0000]this is red text [/color][color=#0000FF]this is blue text[/color][color=#FF0000] this is the remaining red text[/color]

produces:
this is red text this is blue text this is the remaining red text