Engine Crash On Launch

Yesterfay everything was working fine, today morning, when I launch the engine it crashes with following log:

Assertion failed: Existing.Linker->ExportMap[Existing.LinkerIndex].Object==this [File:D:\Build++UE4\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\UObjectLinker.cpp] [Line: 107]

What is going on?


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I’ve gotten the same issue twice now. I found that there were three UIs which were all causing the issue. Compiling any of them or trying to open a level, which BP was referencing them crashed the engine.

I removed the three UIs and re-made them. The issue went away.

Today I closed my project and tried opening it again, and the same issue appeared. Those UIs were not complex in any way. Background image and a few buttons.