[Resolved] Fatal Error after Payment

Both on Subscription and Buy Stripe button, I keep getting WordPress Fatal error:

[31-Mar-2025 12:18:27 UTC] PHP Fatal error:  Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "custom_add_one_day_to_access_end" not found or invalid function name in /data/web/virtuals/373645/virtual/www/wp-includes/class-wp-hook.php:324
Stack trace:
#0 /data/web/virtuals/373645/virtual/www/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array)
#1 /data/web/virtuals/373645/virtual/www/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#2 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/ipn/swpm-stripe-sca-buy-now-ipn.php(317): do_action('swpm_payment_ip...', Array)
#3 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/ipn/swpm-stripe-sca-buy-now-ipn.php(7): SwpmStripeSCABuyNowIpnHandler->handle_stripe_ipn()
#4 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/ipn/swpm-stripe-sca-buy-now-ipn.php(332): SwpmStripeSCABuyNowIpnHandler->__construct()
#5 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/classes/class.swpm-wp-loaded-tasks.php(86): include('/data/web/virtu...')
#6 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/classes/class.swpm-wp-loaded-tasks.php(17): SwpmWpLoadedTasks->swpm_ipn_listener()
#7 /data/web/virtuals/373645/virtual/www/wp-content/plugins/simple-membership/classes/class.simple-wp-membership.php(851): SwpmWpLoadedTasks->do_wp_loaded_tasks()
#8 /data/web/virtuals/373645/virtual/www/wp-includes/class-wp-hook.php(324): SimpleWpMembership->handle_wp_loaded_tasks('')
#9 /data/web/virtuals/373645/virtual/www/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array)
#10 /data/web/virtuals/373645/virtual/www/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#11 /data/web/virtuals/373645/virtual/www/wp-settings.php(726): do_action('wp_loaded')
#12 /data/web/virtuals/373645/virtual/www/wp-config.php(105): require_once('/data/web/virtu...')
#13 /data/web/virtuals/373645/virtual/www/wp-load.php(50): require_once('/data/web/virtu...')
#14 /data/web/virtuals/373645/virtual/www/wp-blog-header.php(13): require_once('/data/web/virtu...')
#15 /data/web/virtuals/373645/virtual/www/index.php(17): require('/data/web/virtu...')
#16 {main}
  thrown in /data/web/virtuals/373645/virtual/www/wp-includes/class-wp-hook.php on line 324

It happens after successful filling in of the Stripe form. I am supposed to be redirected to my Success page. Which does not happen and I see this Fatal error instead.

Can you help out?