Uncaught TypeError: Cannot access offset of type string on string

I just installed the plugin in a test site. Only WooCommerce and Vipps plugins are active. Vipps plugin is in test mode too. Using Twenty Twenty Three theme.

Can place order, but when clicking to checkout, error appears. The log shows

Backend fatal error: PHP Fatal error: Uncaught TypeError: Cannot access offset of type string on string in /home/the-username/domains/the-domain/public_html/wp-content/plugins/woo-vipps/VippsApi.class.php:1371\nStack trace:\n#0 /home/the-username/domains/the-domain/public_html/wp-content/plugins/woo-vipps/VippsApi.class.php(1320): VippsApi->handle_http_response()\n#1 /home/the-username/domains/the-domain/public_html/wp-content/plugins/woo-vipps/VippsApi.class.php(723): VippsApi->http_call()\n#2 /home/the-username/domains/the-domain/public_html/wp-content/plugins/woo-vipps/Vipps.class.php(1568): VippsApi->initiate_checkout()\n#3 /home/the-username/domains/the-domain/public_html/wp-includes/class-wp-hook.php(310): Vipps->vipps_ajax_checkout_start_session()\n#4 /home/the-username/domains/the-domain/public_html/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters()\n#5 /home/the-username/domains/the-domain/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()\n#6 /home/the-username/domains/the-domain/public_html/wp-admin/admin-ajax.php(203): do_action()\n#7 {main}\n thrown in /home/the-username/domains/the-domain/public_html/wp-content/plugins/woo-vipps/VippsApi.class.php on line 1371\n, referer: https://the-domain/vipps_checkout/

I have tested PHP 7 and 8, the same result. Is it because of test mode? Or something else? Thanks.