Shortcodes don’t work (WP v5.2.2)

Hello, I am using your plugin WITHOUT the Ecalypse Theme, and just received this error to my email:

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Chyba typu E_ERROR bola sp?sobená v riadku 330 súboru mywebsite/public_html/wp-content/plugins/ecalypse-rental-starter/class.ecalypse-rental.php. Chybové hlásenie: Uncaught Error: Call to undefined function mb_strtolower() in mywebsite/public_html/wp-content/plugins/ecalypse-rental-starter/class.ecalypse-rental.php:330
Stack trace:
#0 mywebsite/public_html/wp-content/plugins/ecalypse-rental-starter/templates/book-box.php(10): EcalypseRental::t(‘QUICK BOOK’)
#1 mywebsite/public_html/wp-content/plugins/ecalypse-rental-starter/class.ecalypse-rental-theme.php(232): include(‘/var/www/mywebsite/…’)
#2 mywebsite/public_html/wp-includes/shortcodes.php(325): EcalypseRentalTheme::render_book_box(”, ”, ‘ecalypse_rental…’)
#3 [internal function]: do_shortcode_tag(Array)
#4 mywebsite/public_html/wp-includes/shortcodes.php(199): preg_replace_callback(‘/\\[(\\[?)(ecalyp…’, ‘do_shortcode_ta…’, ‘[ecalypse_renta…’)
#5 mywebsite/public_html/wp-includes/class-wp-hook.php(286): do_shortcode(‘[ecalypse_renta…’)
#6 mywebsite/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(‘[eca

I am unable to save a page when the ecalypse shortcode is pasted, however when I press CTRL+S it saves. The page however shows nothing, but a blank table, and the css is broken a bit (may not be relevant).

I am using WordPress v. 5.2.2.

Thanks for any help