I was messing around with Wagon Control to try and get a better idea of how it works, and out of curiousity I placed one beside a requester station. When a train pulled in to make a delivery, a non-recoverable error was triggered. If it helps, the requester station was requesting multiple items, both of which were being delivered on the same train at the time of the crash. Error log below.

The mod Project Cybersyn (1.0.9) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cybersyn::on_train_changed_state (ID 25)
Unknown signal type: capsule
stack traceback:
[C]: in function 'newindex'
__cybersyn/scripts/factorio-api.lua:343: in function 'set_combinator_output'
cybersyn/scripts/layout.lua:294: in function 'set_r_wagon_combs'
cybersyn/scripts/main.lua:708: in function 'on_train_arrives_buffer'
cybersyn/scripts/main.lua:892: in function <cybersyn/scripts/main.lua:882>

this seems to be related to the specific items you were transporting. Can you tell me what they were and what mods they came from?

Just fish and wood, from the base game. (I find them amusing to use when testing in editor).

Tested a little further, I sent a train full of just fish and that caused the crash.

I have found the issue, fish are registered as a special item in the game which I wasn't expecting when programming wagon controls. It will be fixed in the next update. Until then unfortunately you will have to avoid having fish, grenades, and the 2 capsules arrive at request stations with wagon controls.

Not gonna lie, that's kind of hilarious. Thanks for looking into it.