Is this when you use change credential before any boxes with a VC input parameter / scripting ?
I would create a first scriptable box with any VC object has an input and just display its name or something to check if this is because the change credential need the scripting context having access to the VC plug-in to be loaded.
In any case this seems like a bug since it should also be loaded with a different user. In the meantime this may be a work around for you.