See crash log: https://paste.ee/p/mJ8I8 (versions: Minecraft 1.16.2, Fabric API 0.19.0+build.398-1.16, Optifabric 1.4.5, Optifine 1.16.2 HD U G3, Screenshot to Clipboard 1.0.6)

The change that seems to cause the crash is comp500/ScreenshotToClipboard@c9f2280, as Screenshot to Clipboard 1.0.5 works, however the error thrown should have been fixed by 2d72826 (as you mentioned previously "OptiFabric on 1.15.2 won't account for Fabric API's new Mixin to MobEntity").

Installing Replay Mod 1.16.2-2.4.3 stops the crash from occurring, which suggests that this issue (mixin configurations not being applied because there is a mixin to the first class to be applied) was fixed by the workaround proposed in ReplayMod/ReplayMod#327. I then tried adding the dummy prelaunch entrypoint on it's own to a Screenshot to Clipboard jar, and that also fixed the issue. It could be an issue with Fabric-ASM rather than Optifabric or Replay Mod?

I'd almost certainly say it's the exact same problem as Replay Mod, time to try the same fix.