The problem exists also with the JavaScript tracking. The error message on the server side is: "Error while json_decode the Ecommerce items = NULL". It is caused by an invalid JSON format for ec_items. In my case it was because of the escaped double-quote which was needed to insert it into the double-quoted addEcommerceItem values.
I got it working through changing the format I escape double-quotes in names as following:

old: _paq.push(['addEcommerceItem',"1111test","Käse 'test","old "Käse"",10,1]);
new: _paq.push(['addEcommerceItem','1111test','Käse 'test','old "Käse"',10,1]);

What you see in old is the output I get from the shop with htmlspecialchars. I had to use htmlspecialchars_decode() to revert the escaped double-quote and in new you see how I submit the data so that it is working.

Thanks for the report!

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