PHP Fatal error: Uncaught Error: Call to undefined method MCI_Footnotes_Convert

Hello,

since a few weeks I see the following error in all nine websites I run the AMP for WP plugin:

[Fri Apr 16 12:31:17.036494 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: PHP Fatal error:  Uncaught Error: Call to undefined method MCI_Footnotes_Convert::toBool() in /var/www/clients/client1/web434/web/wp-content/plugins/accelerated-mobile-pages/includes/features/functions.php:240
[Fri Apr 16 12:31:17.036514 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: Stack trace:
[Fri Apr 16 12:31:17.036516 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #0 [internal function]: ampforwp_the_content_filter_full()
[Fri Apr 16 12:31:17.036518 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #1 /var/www/clients/client1/web434/web/wp-includes/functions.php(4757): ob_end_flush()
[Fri Apr 16 12:31:17.036520 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #2 /var/www/clients/client1/web434/web/wp-includes/class-wp-hook.php(292): wp_ob_end_flush_all()
[Fri Apr 16 12:31:17.036523 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #3 /var/www/clients/client1/web434/web/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters()
[Fri Apr 16 12:31:17.036525 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #4 /var/www/clients/client1/web434/web/wp-includes/plugin.php(484): WP_Hook->do_action()
[Fri Apr 16 12:31:17.036527 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #5 /var/www/clients/client1/web434/web/wp-includes/load.php(1072): do_action()
[Fri Apr 16 12:31:17.036529 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #6 [internal function]: shutdown_action_hook()
[Fri Apr 16 12:31:17.036531 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr: #7 {main}
[Fri Apr 16 12:31:17.036533 2021] [fcgid:warn] [pid 11487:tid 140696476411648] [client 114.119.163.52:30326] mod_fcgid: stderr:   thrown in /var/www/clients/client1/web434/web/wp-content/plugins/accelerated-mobile-pages/includes/features/functions.php on line 240

Any idea about that?

If you need more information I am happy to provide these.

Kindest regards,
Martin