I found out that the problem was that the code was pasted as a single line which was the cause of the problem. The code was pasted like this:

<script type=”text/javascript”><!–google_ad_client = “pub-xxxxxxxxxxx”;/* 468×15  */google_ad_slot = “xxxxxxxxx”;google_ad_width=468;google_ad_height = 15;//–></script><script type=”text/javascript” src=”http://pagead2.googlesyndication.com/pagead/show_ads.js“></script> 

in one line.

After i edited the code to be several lines, the ads started showing again:

<script type=”text/javascript”>
<!–
google_ad_client = “pub-xxxxxxxxxxx”;
/* 468×15  */
google_ad_slot = “xxxxxxxxx”;
google_ad_width=468;
google_ad_height = 15;
//–>
</script>
<script type=”text/javascript” src=”http://pagead2.googlesyndication.com/pagead/show_ads.js“>
</script>