Valkyrien Skies may use an outdated Mixins version, and running it with Vanilla Fix 1.0.10-150 crashes the game, just as the warning described.
So I report the issue as changelog instructed.
Launcher: HMCL
Forge Version: 2854
crash-2020-12-06_00.59.46-client.txt
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Adding a character to make Vanilla Fix load ahead of Valkyrien Skies can solve the crash problem.
The test was done in a pack that only contained Vanilla Fix and Valkyrien Skies.

check the pinned issues. there's two ways to fix it