Issues with Post 2 Post Plugin

Your error message is discused at this site: http://old.support.advancedcustomfields.com/discussion/5953/declaration-of-acf_taxonomy_field_walkerstart_el/p1

in short they fixed a bug in the addon:

wp-content/plugins/advanced-custom-fields/core/fields/taxonomy.php row/line 419:

function start_el( &$output, $term, $depth, $args = array(), $current_object_id = 0)

wp-content/plugins/advanced-custom-fields/core/fields/taxonomy.php row/line 442:

function end_el( &$output, $term, $depth = 0, $args = array() )