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.

I think this error can be encountered by doing:

1. Have a version installed without the custom_routes config
2. Export configuration based on this version
3. Update module to a version that includes custom_routes, but do not run updates locally. This means you will also not export config with the updates.
4. Deploy updated version of module to site, run database updates
5. Import configuration, which should now revert/overwrite the config put in place by the db update

A quick fix would be to add the custom_routes key to the layout_builder_iframe_modal.settings :

custom_routes: []

then import configuration.