Upon clicking the play button in the launcher, the game attempts to start up, crashing back to the launcher with the following message. I removed all other mods in my mods folder during this test aside from the Fabric API and Pehkui (for Extra Origins).

The game crashed whilst initializing game
Error: java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'origins-classes'!
Exit Code: -1

The crash report is as follows, though strangely, it lists mods other than Origins and its addons despite me having removed the others.

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 10/20/20 12:32 AM
Description: Initializing game

java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'origins-classes'!
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:53)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:423)
at net.minecraft.client.main.Main.main(Main.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:224)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:140)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)
Suppressed: java.lang.NoSuchMethodError: io.github.apace100.origins.power.PowerType.(Ljava/util/function/BiFunction;)V
at moriyashiine.extraorigins.common.registry.EOPowers.(EOPowers.java:25)
at moriyashiine.extraorigins.common.ExtraOrigins.onInitialize(ExtraOrigins.java:9)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient$$Lambda$2255/1355540019.accept(Unknown Source)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50)
... 11 more
Caused by: java.lang.NoSuchMethodError: io.github.apace100.origins.power.PowerType.(Ljava/util/function/BiFunction;)V
at io.github.apace100.originsclasses.power.ClassPowerTypes.(ClassPowerTypes.java:28)
at io.github.apace100.originsclasses.OriginsClasses.onInitialize(OriginsClasses.java:20)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient$$Lambda$2255/1355540019.accept(Unknown Source)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50)
... 11 more

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Render thread
Stacktrace:
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:53)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:423)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.main.Main.main(Main.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:224)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:140)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)

-- System Details --
Details:
Minecraft Version: 1.16.3
Minecraft Version ID: 1.16.3
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1222106720 bytes (1165 MB) / 1723858944 bytes (1644 MB) up to 9544663040 bytes (9102 MB)
CPUs: 12
JVM Flags: 3 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx10G
Fabric Mods:
autoconfig1u: Auto Config v1 Updated 3.2.2
cardinal-components-base: Cardinal Components API (base) 2.6.0
cardinal-components-entity: Cardinal Components API (entities) 2.6.0
cloth-basic-math: Cloth Basic Math 0.5.1
cloth-config2: Cloth Config v4 4.8.1
extraorigins: Extra Origins 1.16.3-1
fabric: Fabric API 0.24.1+build.412-1.16
fabric-api-base: Fabric API Base 0.2.0+9f0fb4b895
fabric-biome-api-v1: Fabric Biome API (v1) 2.0.0+c29459ab95
fabric-blockrenderlayer-v1: Fabric BlockRenderLayer Registration (v1) 1.1.4+c6a8ea8995
fabric-command-api-v1: Fabric Command API (v1) 1.0.8+5ce5339895
fabric-commands-v0: Fabric Commands (v0) 0.2.0+52d3083695
fabric-containers-v0: Fabric Containers (v0) 0.1.8+045df74f95
fabric-content-registries-v0: Fabric Content Registries (v0) 0.1.9+059ea86695
fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.1.2+b7f9825d95
fabric-events-interaction-v0: Fabric Events Interaction (v0) 0.4.1+f8ac1db295
fabric-events-lifecycle-v0: Fabric Events Lifecycle (v0) 0.2.0+16acbe5b95
fabric-game-rule-api-v1: Fabric Game Rule API (v1) 1.0.2+f8ac1db295
fabric-item-api-v1: Fabric Item API (v1) 1.2.0+f8ac1db295
fabric-item-groups-v0: Fabric Item Groups (v0) 0.2.0+438f963695
fabric-key-binding-api-v1: Fabric Key Binding API (v1) 1.0.1+730711c695
fabric-keybindings-v0: Fabric Key Bindings (v0) 0.2.0+3fa9f7c595
fabric-lifecycle-events-v1: Fabric Lifecycle Events (v1) 1.2.0+74cc3b2095
fabric-loot-tables-v1: Fabric Loot Tables (v1) 1.0.1+432ea18895
fabric-mining-levels-v0: Fabric Mining Levels (v0) 0.1.2+b764ce9995
fabric-models-v0: Fabric Models (v0) 0.1.1+f8ac1db295
fabric-networking-blockentity-v0: Fabric Networking Block Entity (v0) 0.2.5+b50ffc7b95
fabric-networking-v0: Fabric Networking (v0) 0.1.10+e00ecb5f95
fabric-object-builder-api-v1: Fabric Object Builder API (v1) 1.9.0+c270b09895
fabric-object-builders-v0: Fabric Object Builders (v0) 0.7.0+432ea18895
fabric-particles-v1: fabric-particles-v1 0.2.2+fb0d9b0e95
fabric-registry-sync-v0: Fabric Registry Sync (v0) 0.6.0+9f0fb4b895
fabric-renderer-api-v1: Fabric Renderer API (v1) 0.3.2+2b46237995
fabric-renderer-indigo: Fabric Renderer - Indigo 0.4.2+2b46237995
fabric-renderer-registries-v1: Fabric Renderer Registries (v1) 2.2.0+f8ac1db295
fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.1.4+f8ac1db295
fabric-rendering-fluids-v1: Fabric Rendering Fluids (v1) 0.1.12+f8ac1db295
fabric-rendering-v0: Fabric Rendering (v0) 1.1.1+f8ac1db295
fabric-rendering-v1: Fabric Rendering (v1) 1.3.1+f8ac1db295
fabric-resource-loader-v0: Fabric Resource Loader (v0) 0.3.1+facf3bbf95
fabric-screen-handler-api-v1: Fabric Screen Handler API (v1) 1.1.0+8724984195
fabric-structure-api-v1: Fabric Structure API (v1) 1.1.0+f8ac1db295
fabric-tag-extensions-v0: Fabric Tag Extensions (v0) 1.0.3+ac8e8c5995
fabric-textures-v0: Fabric Textures (v0) 1.0.4+eae12eb895
fabric-tool-attribute-api-v1: Fabric Tool Attribute API (v1) 1.2.4+2b46237995
fabricloader: Fabric Loader 0.10.2+build.210
fallflyinglib: FallFlyingLib 1.1.0
minecraft: Minecraft 1.16.3
origins: Origins 1.16.3-0.4.0
origins-classes: Origins: Classes 1.16.2-1.0.1
pehkui: Pehkui 1.6.2+1.16.3
reach-entity-attributes: Reach Entity Attribute 1.0.1
Launched Version: fabric-loader-0.10.2+build.210-1.16.3
Backend library: LWJGL version 3.2.2 build 10
Backend API: NO CONTEXT
GL Caps:
Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fabric'
Type: Client (map_client.txt)
CPU:

Hi, sorry this has been hidden a bit in the changelog. The newest version of Origins, 0.4.0, requires all add-ons (including Origins: Classes) to update, due to breaking changes. If you want to play with Origins: Classes and Extra Origins at the moment, you'd need to use version 0.3.8 of Origins.

Thanks for letting me know. Sorry, I never thought to check the file changelog.

Followup: Just tested with a clean install of Fabric 1.16.3 as well, and it led to the same crash.