I believe I've reproduced a bug related to the previous bug on on 2.4.1.

I have the mod configured to prefer stack sizes when possible.

The mod Cybersyn Combinator (2.4.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cybersyn-combinator::on_gui_click (ID 1)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
    [C]: in function '__newindex'
    __cybersyn-combinator__/scripts/gui.lua:623: in function 'handler'
    __flib__/gui.lua:177: in function <__flib__/gui.lua:160>

1. From a fresh empty combinator
2. Click an empty slot
3. Click a signal and confirm the signal
4. While the cursor is flashing for the user to enter a stack quantity, right click and remove the item
5. Click an empty slot

Looks like when clearing (right clicking) a signal, there was some state left behind causing the left click on the next signal button to attempt to reset some properties it couldn't anymore.

Fix will be coming in v2.4.2.

And v2.4.2 is released with the fix.