The website encountered an unexpected error. Try again later. TypeError: implode(): Argument #1 ($array) must be of type array, string given in implode() (line 74 of modules/contrib/layout_builder_iframe_modal/src/Form/LayoutBuilderIframeModalSettingsFor

Did you update from a previous version?
If so, did you run update.php or "drush updb"?

The value is expected to always be an array, either through import of the config via /config/install, or for existing sites, by running database updates. Specifically layout_builder_iframe_modal.post_update.php method layout_builder_iframe_modal_post_update_custom_routes_config() should make this an array, if it isn't already.