Warning on live page - help pleae

Hi there,

As already mentioned in another thread, I get a warning, when I add a certain product to the cart (I opened a new thread, because this warning does not seem to have anything to do with the question asked in the other thread). The product is added, but the the browser does not open the cart page like it should.

This only happens with one product (recordID 13). All other products are OK.

This is the warning:
Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/www.segufixshop.eu/web/detail.php:372) in /var/www/html/www.segufixshop.eu/web/detail.php on line 386

Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/www.segufixshop.eu/web/detail.php:372) in /var/www/html/www.segufixshop.eu/web/detail.php on line 420

I am stumped how this could happen only with one product and not any of the others. I checked for blank spaces, but cannot imagine that to be the problem, because it does only happen with one product. Here are the links:
detail.php?recordID=13
detail.php?recordID=14

I noticed the warning after adding product variations to the price table for record 13. But after uploading the old database again, the warning remained.

Could anyone please help me?