Troubleshooting Hybrid Migration Endpoints in Classic and Modern Hybrid

For anyone reading the comments.
I had Authentication errors 401 even though everything was set up correct.

I had errors in the event viewer system log every time I did iisreset.
This lead me to look into LSA authentication errors. and I fixed all my authentication issues by adding the FQDN of my EWS domain to this registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0 -> BackConnectionHostNames

Steps:
1. Open REGEDIT

2: go to key: -> HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
3. Right click MSV1_0 -> New -> Multi-String Value
4. Enter the name BackConnectionHostNames
5. Right click and select Modify
6. Enter the host you are using for MRSProxy e.g. hybrid.contoso.com
7. Restart IIS


This was my error message for info:

RunspaceId : 189ea21b-b327-40c2-975b-e00cda035dcc
Result : Failed
Message : The connection to the server 'hybrid.contso.com' could not be completed.
ConnectionSettings :
SupportsCutover : False
ErrorDetail : Microsoft.Exchange.Migration.MigrationServerConnectionFailedException: The connection to the
server 'hybrid.contoso.com' could not be completed. --->
Microsoft.Exchange.MailboxReplicationService.RemotePermanentException: The Mailbox Replication
Service was unable to connect to the remote server using the credentials provided. Please check
the credentials and try again. The call to 'https://hybrid.contoso.com/EWS/mrsproxy.svc'
failed. Error details: The HTTP request is unauthorized with client authentication scheme
'Negotiate'. The authentication header received from the server was 'Negotiate,Basic
realm="hybrid.contoso.com"'. --> The remote server returned an error: (401) Unauthorized.. -->
The HTTP request is unauthorized with client authentication scheme 'Negotiate'. The
authentication header received from the server was 'Negotiate,Basic
realm="hybrid.contoso.com"'. --> The remote server returned an error: (401) Unauthorized. --->
Microsoft.Exchange.MailboxReplicationService.RemotePermanentException: The call to
'https://hybrid.contoso.com/EWS/mrsproxy.svc' failed. Error details: The HTTP request is
unauthorized with client authentication scheme 'Negotiate'. The authentication header received
from the server was 'Negotiate,Basic realm="hybrid.contoso.com"'. --> The remote server
returned an error: (401) Unauthorized.. --->
Microsoft.Exchange.MailboxReplicationService.RemotePermanentException: The HTTP request is
unauthorized with client authentication scheme 'Negotiate'. The authentication header received
from the server was 'Negotiate,Basic realm="hybrid.contoso.com"'. --->
Microsoft.Exchange.MailboxReplicationService.RemotePermanentException: The remote server returned
an error: (401) Unauthorized.
--- End of inner exception stack trace ---
--- End of inner exception stack trace ---
--- End of inner exception stack trace ---
at Microsoft.Exchange.MailboxReplicationService.MailboxReplicationServiceFault.<>c__DisplayClas
s97_0.<ReconstructAndThrow>b__0()
at Microsoft.Exchange.MailboxReplicationService.ExecutionContext.Execute(Action operation)
at Microsoft.Exchange.MailboxReplicationService.MailboxReplicationServiceFault.ReconstructAndTh
row(String serverName, VersionInformation serverVersion)
at Microsoft.Exchange.MailboxReplicationService.WcfClientWithFaultHandling`2.<>c__DisplayClass7
_0.<CallService>b__0()
at Microsoft.Exchange.Net.WcfClientBase`1.CallService(Action serviceCall, String context)
at
Microsoft.Exchange.MailboxReplicationService.WcfClientWithFaultHandling`2.CallService(Action
serviceCall, String context)
at Microsoft.Exchange.Migration.MigrationExchangeProxyRpcClient.CanConnectToMrsProxy(Fqdn
serverName, Guid mbxGuid, NetworkCredential credentials, LocalizedException& error)
--- End of inner exception stack trace ---
at Microsoft.Exchange.Migration.DataAccessLayer.ExchangeRemoteMoveEndpoint.VerifyConnectivity()
at Microsoft.Exchange.Management.Migration.MigrationService.Endpoint.TestMigrationServerAvailab
ility.InternalProcessEndpoint(Boolean fromAutoDiscover)
IsValid : True
Identity :
ObjectState : New