Guys the bugs are located in the template file of JoomlaComment (the index.html) and in the comment.class.php file.
In file comment.class.php at lines: 2786 - 2792 the alt texts of the images are misinterpreted to something else. For example I changed line 2786 from:
$html = "<a href='javascript:JOSC_insertUBBTag("b")'><img src='" . $ubbIconList['bold'] . "' class='buttonBB' name='bb' alt='[b]' /></a> ";
to:
$html = "<a href='javascript:JOSC_insertUBBTag("b")'><img src='" . $ubbIconList['bold'] . "' class='buttonBB' name='bb' alt='Bold' /></a> ";
and I did the same for all these lines, I changed the alt texts, since I didn't know where else to look for a solution. This solved almost all the errors I was getting, except one.
One error I couldn't find is when someone makes a "quote" of a comment. The W3C Validator says:
Attribute "cite" exists, but can not be used for this element and this appears to be on the line of the quote beginning with:
<div class='yvBBCode_quote' cite="name">text...
The last fix should be to delete this "cite" attribute from the "div", because this is exactly what causes the error. But where is the cite attribute produced in your code, I got no idea; I looked everywhere, but haven't found it.