On our server we noticed that vanilla accumulators have the positive/negative charged GUI as well, but when clicking it the server crashes with:

Error while running event Cerys-Moon-of-Fulgora::on_gui_switch_state_changed (ID 167)
__Cerys-Moon-of-Fulgora__/scripts/charging-rod.lua:86: attempt to index local 'rod' (a nil value)
stack traceback:
    __Cerys-Moon-of-Fulgora__/scripts/charging-rod.lua:86: in function 'update_rod_lights'
    __Cerys-Moon-of-Fulgora__/scripts/charging-rod.lua:150: in function 'rod_set_state'
    __Cerys-Moon-of-Fulgora__/scripts/charging-rod.lua:412: in function <__Cerys-Moon-of-Fulgora__/scripts/charging-rod.lua:394>

So yeah, everyone has a server-crash button on every vanilla accumulator. :-)

This also crashed my game when toggling the positive/negative GUI with the mod's accumulators on Fulgora, same error

Thanks for the report. Fixed in 4.10.4.

Looks fixed, thanks much! :-)

Correction, 'mostly' fixed, lol.

So when one is close to the server (me) I don't see the GUI on 'other' accumulators at all.

But for people far away (other side of world), they open the gui of a rod on cerys, then open an accumulator gui and they see the gui for just a few frames, they think they might be able to click it fast enough before it vanishes but they didn't want to crash the server again as people were busy. So it seems it's latency dependent... Perhaps keep the rod gui unregistered on them until a control rod is clicked then unregister it when closed? Might take a second to appear on rods then for people far away, but safer? Or any way to just show them on a specific name instead of an overall type? Or perhaps it's just a ghost gui and its interactions will be ignored on invalid accumulator entities anyway I'd hope?

This remaining bug in multiplayer was fixed in v4.11.

Our multiplayer server running 4.11.3 is still having this issue.