Try using fos_userbundle and fr3d_ldapbundle about fr3dldapbundle HOT 1 OPEN

I believe it is related to this (found in the symfony docs for version 3.4):

Deprecated since version 3.4:In previous Symfony versions, firewalls that didn't define their user provider explicitly, used the first existing provider (chain_provider in this example). However, auto-selecting the first user provider has been deprecated in Symfony 3.4 and will throw an exception in 4.0. Always define the provider used by the firewall when there are multiple providers.

So I think you need to configure the chain_provider as the provider for your firewall "main". You have it under form_login, but does it need to be moved so it applies to "main"?

from fr3dldapbundle.