Fatal error: Allowed memory size of 33554432 bytes exhausted

I have been running wordpress for quite some time now and have been able to fix most of issues. But today, I got a strange error that I am unable to fix:

Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 4096 bytes) in /home/mananato/public_html/wp-includes/cache.php on line 505

I have tried to increase the allocated memory by adding the following line to wp-config.php

define('WP_MEMORY_LIMIT', '128M' );

I have tried to add the same line to wp-includes/cache.php without any fruitful results.

So my question is: What is this error? and how can I fix it?

Any help is appreciated.