Cart goes empty when putting in certain products

[Shila]

I've been struggling with this problem for some time now. I think I tried every solution I could find, but the problem keeps getting back.

 

I'm using v 1.5.3.1 with a slightly modified default theme.

 

When you have a cart with random products in it, and put in a certain product, the cart goes empty, either right when you put the product in or if you go to see another page.

 

I found a solution about turning off varnish cache, and at first it seemed to be the thing. But then after a week, I found another product, that would turn the cart empty.

 

Sometimes it works if I change the name or something else about the product and sometimes nothing works and I have to make a duplicate product, to make it work.

 

Can anyone please help me? 

 

 

[Shila]

Additional info: Clearing browser cache, disabeling cache in BO and so on, have no effect.