The error is caused by something in the eSignal software. What exactly we're not sure, but it appears to be something of a timing issue.
This is something that broke around eSignal 8 (eSignal is up to 10.5 I think now). However, you basically just need to click Ok a couple of times and the Equis Data Server and eSignal Data Manager should successfully start.
The only way to avoid seeing this message at this time is to launch the eSignal Data Manager first. Once it is connected, then launch MetaStock.