Shop items adding to cart ends up in eternal waiting loop

Hi, when i click on the "add to cart" button, eternal wait starts.
If i reload the page, the shop item is successfully added to cart.
( to be seen on http://zieltraum.de/store/products/ )

I tried to get an ajax plugin to do my page loads, but as it did not work with appointments+, i deinstalled the plugins again (zajax, ajax page loader)
Now i recognise this behaviour. I can't say if there's a connection to these plugins, but i also have no clue where to look.

Does anyone have an idea or could please help me out?