I expanded it to

$html2pdf->WriteHTML("<style type=\"text/css\">". file_get_contents("contentOA.css"). "</style>" . $htmlbody);
but still got the same error.


Then to test the whole thing without backgrounds, I removed the file_get (there's nothing else but that background in the file anyways) and included one of the backgroundsimages as an IMG with the same path, just to test if the path is correct:

$html2pdf->WriteHTML("<style type=\"text/css\">". "</style>" . $htmlbody);

<div class="feier1">  
	<table style="width: 271px; height: 142px;">  
		<tbody>  
			<tr> <td class="upperleft"> </td><td class="upperhorizon"> </td><td class="upperright"> </td></tr>  
			<tr>  
				<td class="leftvertical"> </td>  
				<td class="eingabefeld"><img alt="" src="template/feier1/lo.png" style="width: 10px; height: 10px;" /> Love message!</td>  
				<td class="rightvertical"> </td>  
			</tr>  
			<tr> <td class="lowerleft"> </td><td class="lowerhorizon"> </td><td class="lowerright"> </td> </tr>  
		</tbody>  
	</table>  
</div>

This works.