Well, this is the source of your homepage:
[code:1] <HTML>
<HEAD>
<TITLE>paardennet</TITLE>
<meta name="keywords" content="paard">
<meta name="description" content="paard">
</HEAD>
<frameset frameborder="0" framespacing="0" border="0" rows="100%,*" noresize>
<frame name="getoutofthissourcecodebastard_frame" src="
213.17.23.45/melis" noresize>
<frame name="getoutofthissourcecodebastard_blank" src="blank.php" marginwidth="0" marginheight="0" noresize>
</frameset>
</HTML>
[/code:1]
As you see you or your hoster or your template is using frames, and maybe IE doesn't like it like that for logins ?
Did you try using this as home-page URL (the real URL) ?:
213.17.23.45/melis/