Error messages started to come

Hi,
Since I updated the MailPoet and MailPoet Premium to latest version 5.9.0 I’m getting the error messages (see below). The site seems to work OK though.
My WordPress version is  6.7.2

What’s wrong? How to fix it?

From: Meditation, Yoga and World Peace [email protected]
Date: 16 Mar 2025, 3:54 PM +0530
To: xxx
Subject: [Meditation, Yoga and World Peace] Your Site is Experiencing a Technical Issue

Howdy!

WordPress has a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.

In this case, WordPress caught an error with one of your plugins, MailPoet.

First, visit your website (https://www.worldpeacesolutions.net/) and check for any visible issues. Next, visit the page where the error was caught (https://www.worldpeacesolutions.net/wp-admin/admin-ajax.php) and check for any visible issues.

Please contact your host for assistance with investigating this issue further.

If your site appears broken and you can’t access your dashboard normally, WordPress now has a special “recovery mode”. This lets you safely login to your dashboard and investigate further.

https://www.worldpeacesolutions.net/

To keep your site safe, this link will expire in 1 day. Don’t worry about that, though: a new link will be emailed to you if the error occurs again after it expires.

When seeking help with this issue, you may be asked for some of the following information:
WordPress version 6.7.2
Active theme: Hueman Child (version 1.0.0)
Current plugin: MailPoet (version 5.9.0)
PHP version 8.0.30

Error Details

An error of type E_ERROR was caused in line 2084 of the file /home/worldp89/public_html/wp-content/plugins/mailpoet/vendor-prefixed/nesbot/carbon/src/Carbon/Traits/Date.php. Error message: Uncaught Error: Call to undefined function mb_strtolower() in /home/worldp89/public_html/wp-content/plugins/mailpoet/vendor-prefixed/nesbot/carbon/src/Carbon/Traits/Date.php:2084
Stack trace:0 /home/worldp89/public_html/wp-content/plugins/mailpoet/vendor-prefixed/nesbot/carbon/src/Carbon/Traits/Date.php(2057): MailPoetVendor\Carbon\Carbon::singularUnit()1 /home/worldp89/public_html/wp-content/plugins/mailpoet/vendor-prefixed/nesbot/carbon/src/Carbon/Traits/Date.php(2207): MailPoetVendor\Carbon\Carbon->setUnit()2 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Newsletter/Sending/ScheduledTasksRepository.php(332): MailPoetVendor\Carbon\Carbon->__call()3 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Cron/Triggers/WordPress.php(156): MailPoet\Newsletter\Sending\ScheduledTasksRepository->findScheduledSendingTasks()4 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Cron/Triggers/WordPress.php(139): MailPoet\Cron\Triggers\WordPress->isSendingQueueActive()5 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Cron/Triggers/WordPress.php(80): MailPoet\Cron\Triggers\WordPress->checkExecutionRequirements()6 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Cron/CronTrigger.php(53): MailPoet\Cron\Triggers\WordPress->run()7 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Config/Initializer.php(472): MailPoet\Cron\CronTrigger->init()8 /home/worldp89/public_html/wp-content/plugins/mailpoet/lib/Config/Initializer.php(354): MailPoet\Config\Initializer->setupCronTrigger()9 /home/worldp89/public_html/wp-includes/class-wp-hook.php(324): MailPoet\Config\Initializer->initialize()10 /home/worldp89/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()11 /home/worldp89/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()12 /home/worldp89/public_html/wp-settings.php(704): do_action()13 /home/worldp89/public_html/wp-config.php(93): require_once(‘/home/worldp89/…’)14 /home/worldp89/public_html/wp-load.php(50): require_once(‘/home/worldp89/…’)15 /home/worldp89/public_html/wp-admin/admin-ajax.php(22): require_once(‘/home/worldp89/…’)16 {main}

thrown