eSync fails when the Ektron Windows Service does not start correctly. Debugging shows an exception reading "Service will retry has error System.InvalidOperationException: Cannot swallow exceptions for methods with non-null return type".
eSync or other service failure with "Cannot swallow" exception in logs.
The Ektron Windows Service reads the web.config files of the sites installed locally in IIS. If one of the URLs in the config file is wrong, the host name cannot be parsed from it, triggering this exception.
Check the WSPath in the web.config for all sites in the local IIS (where the Ektron Windows Service is not starting), fix the URL where necessary. Then stop and start the Ektron Windows Service.
You may also see this error in conjunction with a "Root Exception" during e-Sync or a "No endpoint listening".
If this is the case then ensure that those files are present - if not you will needed to re-create them and ensure the minimum landing pad information is in each of them.