Won't boot when adding second RAM stick

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https://i.redd.it/r7h8vjqg7op31.jpg

I followed the vanilla guide, but I kept getting this error until I removed the RAM stick from slot 0 in my motherboard (has two slots). Both sticks work in slot 1 (so it's not a problem with the physical ram stick), but it will not boot with both sticks in. I've tried using OsxAptioFix3Drv-64, instead of AptioMemoryFix, but the same error occurs. The system boots perfectly (WiFi, Handoff, unlock with Apple Watch) without the second stick of ram, but it just won't work with it. The RAM shows up properly within the BIOS with two sticks and no overclocking is enabled. Has anyone seen this before?

MB: Z390 AsRock Phantom Gaming itx/ac Bios 1.5

CPU: Intel i7 9700K

GPU: PowerColor Vega 64 8GB

RAM: 63 (2x32GB) Corsair Vengeance LPX

EFI that's exactly the same as mine: https://github.com/bydavy/EFI-ASRock-Z390-Phantom-Gaming