This is what I found:
[26-May-2020 09:11:10 UTC] wc_get_order was called incorrectly. wc_get_order should not be called before post types are registered (woocommerce_after_register_post_type action) Backtrace: WP_Fatal_Error_Handler->handle, WP_Recovery_Mode->handle_error, WP_Recovery_Mode_Email_Service->maybe_send_recovery_mode_email, WP_Recovery_Mode_Email_Service->send_recovery_mode_email, wp_mail, PostmanWpMail->send, PostmanWpMail->sendMessage, do_action('post_smtp_on_success'), WP_Hook->do_action, WP_Hook->apply_filters, PostmanEmailLogService->write_success_log, PostmanEmailLogService->writeSuccessLog, PostmanEmailLogService->writeToEmailLog, PostmanEmailLogPurger->truncateLogItems, wp_delete_post, do_action('before_delete_post'), WP_Hook->do_action, WP_Hook->apply_filters, Vendidero\Germanized\Shipments\Validation::before_delete_refund, wc_get_order, wc_doing_it_wrong. This message was added in version 2.5.
[26-May-2020 09:11:10 UTC] wc_get_product was called incorrectly. wc_get_product sollte nicht aufgerufen werden, solange woocommerce_init-, woocommerce_after_register_taxonomy- und woocommerce_after_register_post_type-Aktionen nicht abgeschlossen sind. Backtrace: WP_Fatal_Error_Handler->handle, WP_Recovery_Mode->handle_error, WP_Recovery_Mode_Email_Service->maybe_send_recovery_mode_email, WP_Recovery_Mode_Email_Service->send_recovery_mode_email, wp_mail, PostmanWpMail->send, PostmanWpMail->sendMessage, do_action('post_smtp_on_success'), WP_Hook->do_action, WP_Hook->apply_filters, PostmanEmailLogService->write_success_log, PostmanEmailLogService->writeSuccessLog, PostmanEmailLogService->writeToEmailLog, PostmanEmailLogPurger->truncateLogItems, wp_delete_post, do_action('delete_post'), WP_Hook->do_action, WP_Hook->apply_filters, Atum\Inc\Hooks->before_delete_product, Atum\Inc\Helpers::get_atum_product, wc_get_product, wc_doing_it_wrong. This message was added in version 3.9.
Do you get anything out of that?
Anyway I also set following option in the config.php file: ini_set("memory_limit", "1024M");
The order is now able to load in the backend, but it takes about two minutes. If I deactivate the MI plugin it's there immediately.