Can’t save appearance settings, WordPress error report

Hi,
I’ve tested this plugin and noticed that I could not save the appearance settings. I then got a Email Notification from my Site that an error was caught. I just wanted to report this here, maybe others face the same problem. The Info sent by WordPress is this (I’ve translated some parts):

WordPress-Version 5.8
Current Theme: Storefront (Version 3.7.0)
Current Plugin: Ajax Search Lite (Version 4.9.3)
PHP-Version 7.4.22

Error-Details
==============
An Error of type E_ERROR has been caused on line 98 of file /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings/class/type.class.php  Error message: Uncaught Error: Call to undefined function ctype_digit() in /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings/class/type.class.php:98
Stack trace:
#0 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings/class/type.class.php(132): wpdreamsType->checkData()
#1 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings/class/textsmall.class.php(22): wpdreamsType->getType()
#2 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings/class/type.class.php(34): wpdreamsTextSmall->getType()
#3 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/tabs/instance/general/behavior.php(92): wpdreamsType->__construct()
#4 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/tabs/instance/general_options.php(21): include('/var/www/htdocs...')
#5 /var/www/htdocs/wp-content/plugins/ajax-search-lite/backend/settings.php(72): include('/var/www/htdocs...')
#6 /var/www/htdocs/wp-content/plugins/ajax-search-lite/includes/classes/core/class-asl-menu.php(105): inc