When trying to authenticate to an (oidfed enabled) RP, module oidc complains about
Array to string conversion at /var/www/simplesamlphp-2.3.7/modules/oidc/src/Services/AuthenticationService.php:301
and
Array to string conversion at /var/www/simplesamlphp-2.3.7/modules/oidc/src/Services/src/Services/IdTokenBuilder.php:119
Both lines refer to the claim value as a String, but should be referencing the first value of the array (at least) because internal representation of all attributes are arrays in simpleSAMLphp!
(string)($claims['sub'][0] ?? $user->getIdentifier()),
...
$builder = $builder->relatedTo((string)$claimValue[0]);
Fixing these lines fixes the exceptions and now produce a correct welcoming message at the RP.
I don't understand how this could have escaped your attention, so I'm really curious what I did wrong here?
I'm not sure if this is oidfed RP only?