You need to create a share on the remote server. The E$ is created by default, but it has administrative permissions. If this was working before, then you have had some permissions/account change.

Create a share at that folder, assign permissions as needed, and assign the service account for SQL Server. That is the account that is performing and copying the backup. Alternatively, you could use SQL Agent to run the backup locally and copy it to the share, and then SQL Agent's service account needs the permissions.