Issue with 500 Error Xtheme 5.1.1

We have Xtheme 5.1.1 installed. our site is gethappytax.com . It currently works right now, but if I try to activate any of our other plugins it stops working and gives a blank white screen or 500 error.

Never experienced this before. It doesnt matter what plugin I activate it does it.

You can see what we have activated and not: http://d.pr/i/iEMTpq and http://d.pr/i/fOajRP

Please help this is a big issue.

Hi There,

Thanks for writing in! It seems that you are experiencing PHP memory limit exhaustion. We recommend that you increase your memory allocated to PHP. To increase it, please edit your wp-config.php file and insert these lines:

define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );

You can add it before the line
/*That's all, stop editing! Happy Blogging. */

We would loved to know if this has work for you. Thank you.