WPMUDEV Support, Unfortunately I have reached the limit

Hi WPMUDEV Support,
Unfortunately I have reached the limit of my knowledge with a couple issues which seem to be tied together.

I am using wooCommerce to process payments and memberships ( I know... I tried to use Membership2 but it wouldn't work for me and couldn't be integrated with my tracking system payments).
So, *WooCommerce was working fine but when I updated to WP v.4.3.2 - it seems I am getting DOING_AJAX error. But the error ONLY appears after clicking to process an order from the cart (which should send me to paypal to process payment) - after clicking to order, my site shows these errors (and does not redirect me to paypal):

Notice: Undefined index: DOING_AJAX in /home/paidlink/public_html/wp-content/mu-plugins/grd_functions.php on line 153

Warning: Cannot modify header information - headers already sent by (output started at /home/paidlink/public_html/wp-content/mu-plugins/grd_functions.php:153) in /home/paidlink/public_html/wp-includes/pluggable.php on line 1196

I can't find the problem. My Mu-Plugins were fine before. Not sure what's going on now.
I have Debug on because My WooCommerce was not clicking through to PayPal.

Anyone have any insite?
Maybe someone know this has happened before?

Anything would be a help. All of my work doesn't matter if I can't even process payments properly.

Thanks again for your time.