If you are running core 4.5.10 and connect with clients of versions prior to 4.5.10, then you will get unauthorized (401) response from the core server when requesting internal media.
This is due to a configuration change that only accept authenticated requests for internal media in core.
To fix this, upgrade the client (ImageVault.Client) to version 4.5.10 or later or disable the check on the core service using the following appSetting in the imagevault.core.host.exe.config
<add key="DisableAuthenticationCheckOnInternalMedia" value="true" />
Remember to restart the core service for the changes to take effect.