Fatal error on the frontend
This usually happens when a plugin calls a function that is defined in another plugin without first checking if that function exists, and you have disabled that plugin.
Probably, you can solve this issue by adding this line of code in wp-config.php before the comment /* That’s all, stop editing! Happy blogging. */
define( 'FDP_REMOVE_FILTERS_BEFORE_FIRST_PLUGIN',true );
In any case, please, open a thread on the Support Forum. We would like to know which disabled plugin was causing this issue to inform the authors of that plugin.