Cannot create new custom post types

Whenever I try to create a new custom post type, regardless of slug or name, i get two Messages. The first one says, my new custom post type “xyz” was successfully created, the other one “wrong custom post type specified”. However, no new type is created
I took a look at some stuff in the plugin code (following), and it seems that in the function “wpcf_admin_custom_types_form()” it is looking for the to-be-created-slug in the array of already existing post types. I checked it, and the new slug is definitely passed as GET-parameter, as if I’d want to edit it. This is really frustrating. I looked everywhere in the internet, but nobody seems to have this issue. I wanted to register in the Types forums as well, but registration seems to be disabled.

if ( isset( $_GET['wpcf-post-type'] ) ) {
        $id = sanitize_text_field( $_GET['wpcf-post-type'] );
    } elseif ( isset( $_POST['wpcf-post-type'] ) ) {
        $id = sanitize_text_field( $_POST['wpcf-post-type'] );
    }

    if ( $id ) {
        $custom_types = get_option( 'wpcf-custom-types', array() );
        if ( isset( $custom_types[$id] ) ) {
            $ct = $custom_types[$id];
            $update = true;
            // Set rewrite if needed
            if ( isset( $_GET['wpcf-rewrite'] ) ) {
                flush_rewrite_rules();
            }
        } else {
            wpcf_admin_message( __( 'Wrong custom post type specified', 'wpcf' ), 'error' );
            return false;
        }
    } else {
        $ct = wpcf_custom_types_default();
    }

https://ww.wp.xz.cn/plugins/types/