Is the package in question loaded by RServe when it starts?
This note has the following to say:
Running R expressions from Rserv.cfg
You can use your Rserv.cfg file to preload packages that you would otherwise have to load from your scripts. To do this, add the keyword
evalto your configuration file and follow it with an expression. For example, to load the mvoutlier library and create an object x and set its value to 10, add a line like the following.
eval library(“mvoutier”); x = 10;