Loading (well, reloading) workflows that include Visual Basic expressions in a WCF hosted service is not as straightforward as one might expect. If you have created your XAML workflow based on an ActivityBuilder or a DynamicActivity then the Visual Basic settings aren’t configured automatically.
Anyone who’s done much in the way of programming in Windows Communications Foundation (WCF) will be well aware of the problems that can occur due to even minor configuration inconsistencies between client and service endpoint settings. The fact those settings are normally defined within application configuration files means that a typo, or one or more settings that are different between files can grind connectivity to a halt.