Hi,
I think I have an idea of what's happening, although I'm not sure how your engine ended up in this state.
In the Reference Viewer for T_DefaultMaskTexture, I noticed it is being referenced by DefaultTexture (the redirector, not the actual texture). Redirectors are commonly created when an asset is moved, so Unreal leaves a reference behind to preserve existing links, or when an asset is deleted and its references are redirected to another asset. My guess is that the engine's DefaultTexture was somehow deleted, moved, or replaced, and its references ended up pointing to T_DefaultMaskTexture, which isn't suitable for that texture slot.
In my project, only the skin materials on my model use a diffuse/base color texture, since they're the only materials with painted color. That would explain why only those materials are rendering incorrectly.
To confirm this, could you try unchecking the Use Diffuse switch in one of the affected material instances? I'm fairly confident the compilation error will disappear if that's the cause.
For reference, the engine's DefaultTexture should look like this:
I think it's worth restoring that texture to avoid any future issues. One thing you can try is verifying your Unreal Engine installation. In the Epic Games Launcher, click the arrow next to the Launch button for your engine version and choose Verify. This will check the integrity of the engine files and should restore the original
DefaultTexture if it has been modified or replaced.