I think this has already happened before. Started again after update.
On of/on labels via hotkey:

The mod Utilization Monitor (Blargh) (0.8.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UtilizationMonitorBlargh::toggle-utilization-monitor-labels (ID 312)
The mod Schall Endgame Evolution (1.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SchallEndgameEvolution::on_runtime_mod_setting_changed (ID 59)
SchallEndgameEvolution/control.lua:98: bad argument #3 of 3 to 'index' (string expected, got nil)
stack traceback:
[C]: in function '__index'
__SchallEndgameEvolution/control.lua:98: in function <SchallEndgameEvolution/control.lua:97>
stack traceback:
[C]: in function 'newindex'
__UtilizationMonitorBlargh/control.lua:624: in function <UtilizationMonitorBlargh/control.lua:623>

This is caused by the other mod, SchallEndgameEvolution. It assumes assume player_index is defined in the on_runtime_mod_setting_changed event (and they're watching EVERY setting change, instead of filtering to their own...). However, https://lua-api.factorio.com/latest/events.html#on_runtime_mod_setting_changed explicitly says this isn't true in the case of a script changing a setting (which UMB is doing - it's trying to update the mod setting tick box to match what you're toggling).

The problem is in SchallEndgameEvolution, line 98:

local function runtime_mod_setting_changed(e)
  local player = game.players[e.player_index]

FYI, I posted to that mod at https://mods.factorio.com/mod/SchallEndgameEvolution/discussion/601a12f0d395153dd3917ae3 as well.

I saw that it had something to do with another mod, I reported it here because this surfaced after UMB update. Thanks for reporting it for the other mod.

Yeah, 0.8.5 triggered it because the code is fixing a bug in 0.8.4 (the operating state gets out of sync with the checkbox in the Mod Settings page), heh.

Sorry for the waiting. Finally have some time for Factorio modding.

Thanks for reporting.
I was not aware that can be optional, and was not expecting mod settings being changed by script.
Just released 0.17.16 / 0.18.8 / 1.0.2 / 1.1.1, which (hopefully) fixed the problem.
I will update my other mods (if they do anything with runtime_mod_setting_changed and have same problem) in a few days.