Hi Nikola,
I have used your plugin for the last three years. Today I encountered a Fatal error message while responding to appointment e-mail. As the appointment was supposed to transfer from "pending" to "confirmed".
Please see below:

Fatal error: Uncaught Error: Call to undefined function WC() in /home/atgrace1/public_html/wp-content/plugins/easy-appointments-connect/src/woo_logic.php:205 Stack trace: #0 /home/atgrace1/public_html/wp-includes/class-wp-hook.php(287): EAC_Woo_Logic->process_appointment('1169', Array) #1 /home/atgrace1/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array) #2 /home/atgrace1/public_html/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #3 /home/atgrace1/public_html/wp-content/plugins/easy-appointments/src/mail.php(211): do_action('ea_new_app', '1169', Array) #4 /home/atgrace1/public_html/wp-includes/class-wp-hook.php(287): EAMail->parse_mail_link(Object(WP)) #5 /home/atgrace1/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array) #6 /home/atgrace1/public_html/wp-includes/plugin.php(544): WP_Hook->do_action(Array) #7 /home/atgrace1/public_html/wp-includes/class-wp.php(758): do_action_ref_array('wp', Array) #8 /home/atgrace1/public_html/wp-includes/functions.php in /home/atgrace1/public_html/wp-content/plugins/easy-appointments-connect/src/woo_logic.php on line 205 There has been a critical error on your website. Learn more about debugging in WordPress.