I know there are similair topics on this forum, but they were no help. So I started this one since I am unable to post comments. First of some specs of my installation.

Mysql version 4.1.21
Database utf8_general_ci
PHP version 5.2.4
Webserver Apache/2.0.46 (CentOS)
Joomla! 1.5.15 DutchJoomla! Stable [ Wojmamni Ama Mamni ]
Joomlacomment 4.0 beta 2

Could it have something to do with my hoster running Mysql 4?

When I run in debug mode, I get the following which ends with a popup saying: 'Ajaxready: KO ! Status=500'

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###AJAXSEND:
##URL=http://www.staalenco.nl/index.php ?option=com_comment&no_html=1&component=&joscsectionid=4&josctask=ajax_insert&comment_id=-1&content_id=198&tname=Alexander%20Staal&temail=automatic&twebsite=&ttitle=fhfgh&tcomment=ghdfhgfhgfhhgfhh
##onReadyStateChange=function JOSC_editPostResponse() {
if (JOSC_ajaxReady()) {
if (JOSC_http.responseText.indexOf("invalid" == -1) {
var form = document.joomlacommentform;
var element = document.getElementById("Comments";
var xmlDocument = JOSC_getXmlResponse(true);
if (!xmlDocument) {
return 0;
}
var id = JOSC_xmlValue(xmlDocument, "id";
var captcha = JOSC_xmlValue(xmlDocument, "captcha";
if (captcha) {
JOSC_refreshCaptcha(captcha);
if (id == "captchaalert" {
alert(_JOOMLACOMMENT_FORMVALIDATE_CAPTCHA_FAILED);
return 0;
}
if (id == "captcha" {
return 0;
}
}
anchor = "josc" + id;
var idsave = id;
id = "post" + id;
var body = JOSC_xmlValue(xmlDocument, "body";
var post = document.getElementById(id);
var after = JOSC_xmlValue(xmlDocument, "after";
JOSC_clearInputbox();
var noerror = JOSC_xmlValue(xmlDocument, "noerror";
if (noerror == 0) {
alert(_JOOMLACOMMENT_REQUEST_ERROR);
form.tcomment.value = JOSC_http.responseText;
return 0;
}
var published = JOSC_xmlValue(xmlDocument, "published";
if (published == 0) {
alert(_JOOMLACOMMENT_BEFORE_APPROVAL);
form.tcomment.value = "";
if (self.JOSC_afterAjaxResponse) {
JOSC_afterAjaxResponse("response_approval";
}
return 0;
}
if (post) {
var className = JOSC_getPostClass(post);
var indent = post.style.paddingLeft;
JOSC_insertAdjacentHTML(post, "beforeBegin", body);
JOSC_removePost(post);
newPost = document.getElementById(id);
JOSC_setPostClass(newPost, className);
newPost.style.paddingLeft = indent;
JOSC_modifyForm(_JOOMLACOMMENT_WRITECOMMENT, _JOOMLACOMMENT_SENDFORM, function (event) {JOSC_editPost(-1, -1);});
form.tname.value = JOSC_userName;
if (document.getElementsByName("temail"[0]) {
form.temail.value = JOSC_userEmail;
}
if (document.getElementsByName("website"[0]) {
form.website.value = JOSC_userWebsite;
}
if (self.JOSC_afterAjaxResponse) {
JOSC_afterAjaxResponse("response_editpost";
}
} else {
if (!after || after == -1) {
if (JOSC_sortDownward != 0) {
if (JOSC_postREFRESH) {
JOSC_refreshPage(_JOOMLACOMMENT_MSG_NEEDREFRESH, idsave);
} else {
JOSC_insertAdjacentHTML(element, "afterBegin", body);
}

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Anybody? I already set SH404SEF to ignore Joomlacomment.