Started getting the error this morning, also can't pull up the settings.

Message: Interface\AddOns\ls_Toasts\init.lua:311: Attempt to unregister unknown event "ACHIEVEMENT_EARNED"
Time: Thu Sep 5 12:16:11 2019
Count: 1
Stack: Interface\AddOns\ls_Toasts\init.lua:311: Attempt to unregister unknown event "ACHIEVEMENT_EARNED"
[C]: in function UnregisterEvent' Interface\AddOns\ls_Toasts\init.lua:311: in function func'
Interface\AddOns\ls_Toasts\core\core.lua:42: in function <Interface\AddOns\ls_Toasts\core\core.lua:40>

Locals: (*temporary) = AlertFrame {
0 =
SetSubSystemAnchorPriority = defined @interface\FrameXML\AlertFrames.lua:350
BuildQuestData = defined @interface\FrameXML\AlertFrames.lua:632
shouldQueueAlertsFlags =

{
}
SetJustification = defined @interface\FrameXML\AlertFrames.lua:377
CreateSubSystem = defined @interface\FrameXML\AlertFrames.lua:311
CreateQueuedSubSystem = defined @interface\FrameXML\AlertFrames.lua:328
SetEnabledFlag = defined @interface\FrameXML\AlertFrames.lua:277
SetVariablesLoaded = defined @interface\FrameXML\AlertFrames.lua:294
GetPointsForJustification = defined @interface\FrameXML\AlertFrames.lua:402
SetAlertsEnabled = defined @interface\FrameXML\AlertFrames.lua:299
BuildLFGRewardData = defined @interface\FrameXML\AlertFrames.lua:590
AreAlertsEnabled = defined @interface\FrameXML\AlertFrames.lua:303
AddQueuedAlertFrameSubSystem = defined @interface\FrameXML\AlertFrames.lua:338
AddAlertFrameSubSystem = defined @interface\FrameXML\AlertFrames.lua:342
GetJustification = defined @interface\FrameXML\AlertFrames.lua:384
anchorPrioritiesDirty = true
OnLoad = defined @interface\FrameXML\AlertFrames.lua:417
CleanAnchorPriorities = defined @interface\FrameXML\AlertFrames.lua:360
AddExternallyAnchoredSubSystem = defined @interface\FrameXML\AlertFrames.lua:318
ShouldSupressDungeonOrScenarioAlert = defined @interface\FrameXML\AlertFrames.lua:658
OnEvent = defined @interface\FrameXML\AlertFrames.lua:423
AddAutoAnchoredSubSystem = defined @interface\FrameXML\AlertFrames.lua:323
BuildScenarioRewardData = defined @interface\FrameXML\AlertFrames.lua:622
AddSimpleAlertFrameSubSystem = defined @interface\FrameXML\AlertFrames.lua:332
SetPlayerEnteredWorld = defined @interface\FrameXML\AlertFrames.lua:289
alertFrameSubSystems = {
}
UpdateAnchors = defined @interface\FrameXML\AlertFrames.lua:368
ignoreFramePositionManager = true
AddAlertFrame = defined @interface\FrameXML\AlertFrames.lua:409
}
(*temporary) = "ACHIEVEMENT_EARNED"

I figured it out, I'm playing Classic and it seems that the Twitch client mistakenly updated to the version meant for vanilla rather than classic.

Good that you figured it out, CF's and Twitch client's Classic support leaves a lot to be desired.

Addon dev discord and IRC servers are filled w/ complaints atm T_T Even if you, as a dev, do everything according to curse/twitch guidelines, there's still a chance that the twitch client will pull the wrong version.