I had my webhost increase the memory to 128 Meg and now the error has gone away. I tried all the other suggestions above before memory increase and it never helped. I believe its related to the amount of memory that you have free. I use an add-on called Server IP and Memory Usage (http://apasionados.es/#utm_source=wpadmin&utm_medium=plugin&utm_campaign=server-ip-memory-usage-plugin) and before with 64meg I had less then 10% free. Now I have 60%. Also I set the max memory that Wordfence to use to 20Meg but during a scan it says it used over 32Meg of memory to scan.This limit really does nothing for me. Oddly Active says he has 255Meg and still gets the error. How much memory do you have free?
Thanks,
Steve