Grammatically, this is not a problem, because nonrestrictive relative clauses like
which forwards it to ModuleB.
modify the NP they precede, and thus which, the coreferential relative pronoun subject of forwards, must refer to ModuleA, which immediately precedes it. Couldn't be much clearer.
However, this is not a grammatical problem, but a potential reading problem,
and that's caused by being confusing, not by being ungrammatical.
The server picks up the emitted fax via ModuleA, which forwards it to ModuleB.
Consider how someone who doesn't know exactly what you (the author) do
understands this sentence when they read it.
There is an arbitrarily-named (Server) non-human actor widget involved in some action of picking up via and/or emitting and/or passing (the distinction isn't clear here, though no doubt it's clear in context) some arbitrarily-named information thingie (Fax) to or via (which means through; hmmm) some other arbitrarily-named (ModuleA) non-human actor widget, maybe with some third arbitrarily-named (ModuleB) non-human actor widget standing around with something to do, like catch passed faxes.
(If you've noticed all the players, that is, and if you can imagine them on set with a script, and if you haven't confused this arbitrarily-named non-human actor widget with one of the other arbitrarily-named non-human actor widgets, maybe the first or the second.)
You see how confusing it can be, and how much you're asking of the reader.
That's the result of two things:
- the implicit
A follows B temporally structure of the sentence,
which is not obvious and needs reinforcement, like adding then after which
The server picks up the emitted fax via ModuleA, which then forwards it to ModuleB.
- using via to mark ModuleA, which makes it unclear whether ModuleA is
- just a part of Server;
i.e, a tool,
- a different kind of thingie that can do things;
i.e, an actor.
Executive summary: Readers may discount obvious grammatical relation, if they're unclear on the status of the items involved. This is especially true for metaphoric descriptions, and all computer terms are metaphoric.