So I solved the problem now. Although I do not think this is a very neat solution, perhaps this is the only way.
I just realized that I would never get numbers with more than 12 digits only when PHPExcel is wrong. Therefore, I overtook all my floats to 12 digits using number_format:
if ((is_numeric($sTempValue))&&(strpos($sTempValue,'.')))
{
$sTempValue = rtrim(rtrim(number_format($sTempValue,12,',',''),'0'),',');
}