I performed a fresh install of 155 on my test machine with the test data and it has the same problem on that.
I was using the responsive classic template on that. I have tried classic green and get the same issue goes to index.php?main_page=time_out&zenid=co24rosqf910i1i5eg8u9r9i37 when trying to add to cart. the links all have &zenid=g3a5efvifobos3q6g2ojcokq75 so will have same issue.

For the live site I started life as 1.3.0 and has been upgraded through all the releases to 1.5.4 it now uses a modified responsive sheffield blue template.