Hi @Frederico Gil ,
It looks like you are getting the "Invalid Credentials" error right after the other error. Please make sure that you are entering the valid domain user credentials and that the user has read access and MFA enabled. Is the GA account in a federated domain and does the GA account have MFA enabled? Try to check on sign-in logs to see if the Cloud connector account is having any restriction on logging in
If this does not help, one thing you can try is to create separate enterprise admins for all domains to restore the access.
Another possibility is that the AD connect server is not able to reach the Active directory domain controllers. This could be because of incorrect routing or because you have ports blocked on the network. Make sure traffic is allowed on ports documented at https://docs.microsoft.com/en-us/azure/active-directory/hybrid/reference-connect-ports#table-1---azure-ad-connect-and-on-premises-ad
If you still face this issue after checking these variables, let me know.