The mod Rail Signal Planner (2.1.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RailSignalPlanner::rsp-open-menu (ID 257)
RailSignalPlanner/scripts/shortcuts.lua:4: attempt to call global 'toggle_signal_ui' (a nil value)
stack traceback:
RailSignalPlanner/scripts/shortcuts.lua:4: in function <RailSignalPlanner/scripts/shortcuts.lua:3>

+1

Yup, settings button is missing. All of my signals are hard stuck at 3-length trains.

+1 not able to configure tile gaps like before.

Workaround till author updates.
Remove the current mod.
Download the latest 2.0 version of the mod
Go to where your mods are.
Move the contents of the RailSignalPlanner.zip - the whole folder basically - into the mods folder where the mods are stored.
Go into the RailSignalPlanner folder and edit the info.json file.
edit the version from 2.0.3 to 2.1.4
edit the factorio_version from 2.0 to 2.1

I already have the settings as recommended, but there is simply no button.

info.json:
{
"name": "RailSignalPlanner",
"version": "2.1.4",
"title": "Rail Signal Planner",
"author": "LeonSkills",
"contact": "discord: L__eon",
"homepage": "https://github.com/LeonSkills/FactorioMods",
"factorio_version": "2.1",
"dependencies": [
"base >= 2.1"
],
"description": "Provides a tool to automatically place chain and rail signals on rails and intersections."
}