SimpleSAMLphp as idp : Metadata not found

I am working on SSO authentification system using SAML2 protocol. Since i do not have the real IDP informations yet, i needed test environment so i used this docker image which worked like a charm. I am able to access it through localhost:8080 without any problem and also to log in with test account.

But then, when i try to log into my SP (the website i am working on), though i am well redirected to the idp at localhost:8080/simplesaml/saml2/idp/SSOService.php, i got an error telling me metadata not found. here is the stack trace :

SimpleSAML_Error_MetadataNotFound: METADATANOTFOUND('%ENTITYID%' => '\'\'')

Backtrace:
    3 lib/SimpleSAML/Metadata/MetaDataStorageHandler.php:300 (SimpleSAML_Metadata_MetaDataStorageHandler::getMetaData)
    2 lib/SimpleSAML/Metadata/MetaDataStorageHandler.php:320 (SimpleSAML_Metadata_MetaDataStorageHandler::getMetaDataConfig)
    1 modules/saml/lib/IdP/SAML2.php:330 (sspmod_saml_IdP_SAML2::receiveAuthnRequest)
    0 www/saml2/idp/SSOService.php:19 (N/A)

Did anybody ever faced the same one ? Because i don't really understand where SimpleSAMLphp try to get those metadata.

NOTE :

I am manually able to find metadata at localhost:8080/simplesaml/saml2/idp/metadata.php
It is an old project i am working for and it is a fork of symfony 1 so i can't use more recent plugins. So for SSO i use this plugin on service provider side.

Solution

Finally solved. The Authn Request i sent contained empty "issuer" field and this one was needed on idp side since simpleSAMLphp seemes to store SP Metadata in an array like Metdata['spName']. So of course their was no entry for the empty string as key.