Settings Page appears blank

Hello everyone,

I have installed succesfully this plugin to a lot of sites but in the one I installed it doesn’t work.
Settings Page appears blank but from the shortcut of the admin’s ribbon I can turn it on and off but I cannot edit anything else.
I received the following notification in my email

An error of type E_ERROR was caused in line 1455 of the file /home/wtfotfwx/fotisp.gr/wp-content/plugins/under-construction-page/under-construction.php. Error message: Uncaught Error: Call to undefined method Weglot\Client\Api\LanguageEntry::getIso639() in /home/wtfotfwx/fotisp.gr/wp-content/plugins/under-construction-page/under-construction.php:1455
Stack trace:
#0 /home/wtfotfwx/fotisp.gr/wp-content/plugins/under-construction-page/under-construction.php(2061): UCP::tab_content()
#1 /home/wtfotfwx/fotisp.gr/wp-includes/class-wp-hook.php(303): UCP::main_page('')
#2 /home/wtfotfwx/fotisp.gr/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array)
#3 /home/wtfotfwx/fotisp.gr/wp-includes/plugin.php(470): WP_Hook->do_action(Array)
#4 /home/wtfotfwx/fotisp.gr/wp-admin/admin.php(259): do_action('settings_page_u...')
#5 /home/wtfotfwx/fotisp.gr/wp-admin/options-general.php(10): require_once('/home/wtfotfwx/...')
#6 {main}
  thrown

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