Getting error when I put short code on page

I am getting this error when loading the page:

Warning: array_filter() expects parameter 1 to be array, string given in /var/www/wp-content/plugins/pie-register/pie-register.php on line 683

Notice: unserialize(): Error at offset 0 of 1 bytes in /var/www/wp-content/plugins/pie-register/pie-register.php on line 1059

And this error when attempting to edit the page:

Warning: Illegal string offset ‘form’ in /var/www/wp-content/plugins/pie-register/pie_register_template/registration/registration_form_template.php on line 56

Fatal error: Uncaught Error: Cannot use string offset as an array in /var/www/wp-content/plugins/pie-register/pie_register_template/registration/registration_form_template.php:56 Stack trace: #0 /var/www/wp-content/plugins/pie-register/pie_register_template/registration/registration_form_template.php(1311): Registration_form_template->addFormData(‘true’, ‘true’) #1 /var/www/wp-content/plugins/pie-register/pie-register.php(5939): Registration_form_template->printFields(false, ‘1’, ‘true’, ‘true’) #2 /var/www/wp-content/plugins/pie-register/pie-register.php(5989): PieRegister->outputRegForm(false, ‘1’, ‘true’, ‘true’) #3 /var/www/wp-content/plugins/pie-register/pie-register.php(1166): PieRegister->showForm(‘1’, ‘true’, ‘true’) #4 /var/www/wp-includes/shortcodes.php(325): PieRegister->piereg_registration_form(Array, ”, ‘pie_register_fo…’) #5 [internal function]: do_shortcode_tag(Array) #6 /var/www/wp-includes/shortcodes.php(199): preg_replace_callback(‘/\\[(\\[?)(pie_re…’, ‘do_shortcode_ta…’, ‘[pie_register_f…’) #7 in /var/www/wp-content/plugins/pie-register/pie_register_template/registration/registration_form_template.php on line 56
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I’ve deactivated all plugins, it doesn’t fix the issue.

Any leads would be very much appreciated!

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