Hello there @LordMegatron666!
Checking around the community, there are cases of certain features causing problem when migrating from 5.6 to 5.7, yet I’m not seeing many like yours.
Reviewing your log, it looks like a null crash. The engine is looking for a pointer or object that doesn’t exist, or it’s corrupted. And it seems to be happening during the streaming phase of the build. Meaning, it’s possible that UE is failing to read your landscape material, or the landscape itself.
So, this is mainly related to the migration, as you changed nothing from your landscape. There are a few methods to test post migration, let’s cover them one by one:
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First, since your landscape data was baked in 5.6, it needs a rebuild from 5.7. Go to Build, and click on “Build Landscape”. You can also force a refresh by selecting your landscape actor, making a minor change, then re-save all.
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Another thing to check, if your landscape is using Nanite, try toggling it off and on to force a rebuild. Please do the same for HLODs, if you are using them in your project.
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For the possible material conflict, I would remove the material from the landscape, reimport/refresh it, then re-assign it back to the landscape. As an extra test, go to Project Settings > Rendering > Textures, and disable “Texture Streaming”.
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If the crash persists, there could be stale/corrupted data from your previous version still lingering in the cache. To clear that up, go to your project’s main directory, and delete folders DerivedDataCache, Intermediate, and Saved. After that, re-open your project, and allow a full shader recompile.