I have not seen this error before, had to research it. This error, in regards to WordPress, seems to be related to PHP exhausting the memory allocated to it or a plugin/theme conflict.
My first suggestion is to try increasing the memory allocated to PHP. Here’s an excellent post on to do it. It outlines several methods. The one that will work for you will depend on your web host. I suggestion starting at the top and working your way done the list.
If that does not work, try looking for a plugin/theme conflict. Here are the links to the FAQs.
Hope that helps, let me know.