It simply means WordPress can not allocate more space in memory(RAM) to execute or continue running process. So try increasing the memory allocation for PHP. See these relevant posts.
As described in the above posts, you need to manually edit the PHP file. However, the plugin has a setting for it. Go to Appearance -> Responsive Column Widgets -> General Options -> Option Management -> Attempt to Override Allocated Memory Size, and set enough size there. I suppose 128MB should be enough.
Also, you should upgrade your PHP to 5.3 or above if you are using a lesser version. It has better memory handling and I've seen many occasions that dramatically decrease the memory usages.