I found a solution and identified my specific issue (I think the same error could be caused by other issues).
For some reason, almost all of my Pose Assets (red ones) and some Animation Sequences, which are standard and come with Metahuman, were broken. They were used in my standard Post Process Animation Blueprint “m_med_nrw_animbp,” which hooked up the main skeleton and was linked to all Metahumans (which is why almost any action with Metahumans involving loading animations and poses for this skeleton into memory caused the engine to crash). I realized they were broken simply by moving the sliders in them (to change the pose), and I saw strong, incorrect distortions in the sequence that clearly shouldn’t be there, or most likely shouldn’t be there.
I did two main steps to fix this, but perhaps even one would have helped.
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The simplest possible solution (which definitely fixed all the broken Animation Seamlines for me, but it doesn’t necessarily fix all the Pose Assets): go to the main skeleton for Metahuman (I only had one skeleton, metahuman_base_skel), click the Retarget Manager button at the top, and in the manager window that appears, click Update All + Save.
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To fix all the Pose Assets, I simply forced a full update of these poses. I first mass-filtered them all for the male Metahuman by Pose Asset (then did the same for the female), selected them, right-clicked, and chose Edit Selection In Property Matrix. You need to change something significant there to force a full update without breaking them completely. In one version, I saw that they were all using the same Retarget Source Asset. So, I made a copy of it and mass-replaced the original Retarget Source Asset with the copy, and then deleted the copy. Then they all updated and didn’t break or change. But in the second version for the female skeleton, I had “None” in the Retarget Source Asset. So I simply entered “Default” in the Retarget Source field and saved them all.
And so they all updated and fixed themselves. Because all my skeletons were correct, but apparently I messed up the retargeting at some point and changed the Metahuman skeleton, and then reverted it to its original state. But apparently all the related assets didn’t update and remained broken and incorrect, which caused the engine to crash when loading them into memory (because it was looking for the wrong skeleton or something). Although, visually, I didn’t see anything broken in any of my Metahumans because I don’t use those poses and animations.
UPDATED: Ah, no, that didn’t fix the crashes. The problem turned out to be something else. It turns out that in my custom animation blueprint, I used the “Extract Bone Transform from Animation At Time” node in the animation blueprint’s initialization event to extract Pelvis’s position at the end of a specific “Animation Sequence.” And it was precisely because of this node in the initialization that I was getting this crash with this error.
This happened during compilation because it was re-initializing. And when switching Animation Mode, it happened because my animation blueprint was selected by default, which also caused it to initialize for any Metahuman.
The final solution in my case: In my animation blueprint, I simply moved the “Extract Bone Transform from Animation At Time” node from Initialization to Begin Play, updated all Pose Assets, and rolled back the broken Skeletal Mesh of my Metahuman (it somehow broke while I was trying all this). And then everything worked as it should, correctly and without crashes.