This issue has not occurred in previous versions and seems to occur from 4.27.
LogWindows: Error: === Critical error: === LogWindows: Error: LogWindows: Error: Assertion failed: Header [File:G:\UE4_27\Engine\Source\Runtime\MovieScene\Public\EntitySystem/MovieSceneEntitySystemTypes.h] [Line: 612] LogWindows: Error: LogWindows: Error: LogWindows: Error: LogWindows: Error: [Callstack] 0x00007ffda34e4f99 KERNELBASE.dll!UnknownFunction [] LogWindows: Error: [Callstack] 0x00007ffcb8720316 UE4Editor-Core.dll!ReportAssert() [G:\UE4_27\Engine\Source\Runtime\Core\Private\Windows\WindowsPlatformCrashContext.cpp:1644] LogWindows: Error: [Callstack] 0x00007ffcb8724318 UE4Editor-Core.dll!FWindowsErrorOutputDevice::Serialize() [G:\UE4_27\Engine\Source\Runtime\Core\Private\Windows\WindowsErrorOutputDevice.cpp:78] LogWindows: Error: [Callstack] 0x00007ffcb844252d UE4Editor-Core.dll!FOutputDevice::LogfImpl() [G:\UE4_27\Engine\Source\Runtime\Core\Private\Misc\OutputDevice.cpp:61] LogWindows: Error: [Callstack] 0x00007ffcb8396b45 UE4Editor-Core.dll!AssertFailedImplV() [G:\UE4_27\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:104] LogWindows: Error: [Callstack] 0x00007ffcb83981a0 UE4Editor-Core.dll!FDebug::CheckVerifyFailedImpl() [G:\UE4_27\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:461] LogWindows: Error: [Callstack] 0x00007ffcac00e63b UE4Editor-MovieSceneTracks.dll!UE::MovieScene::TInitialValueProcessorImpl<UE::MovieScene::TIndirectPropertyTraits<FTransform,UE::MovieScene::FIntermediate3DTransform>,UE::MovieScene::TPropertyMetaData<>,TIntegerSequence<int> >::VisitAllocationCached() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Public\EntitySystem\MovieScenePropertyComponentHandler.h:154] LogWindows: Error: [Callstack] 0x00007ffcabf9ae58 UE4Editor-MovieSceneTracks.dll!UE::MovieScene::FInitialValueMutation::InitializeAllocation() [G:\UE4_27\Engine\Source\Runtime\MovieSceneTracks\Private\Systems\MovieSceneInitialValueSystem.cpp:69] LogWindows: Error: [Callstack] 0x00007ffcac22de0a UE4Editor-MovieScene.dll!UE::MovieScene::FEntityManager::MutateAll() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntityManager.cpp:1647] LogWindows: Error: [Callstack] 0x00007ffcabfa80f8 UE4Editor-MovieSceneTracks.dll!UMovieSceneInitialValueSystem::OnRun() [G:\UE4_27\Engine\Source\Runtime\MovieSceneTracks\Private\Systems\MovieSceneInitialValueSystem.cpp:130] LogWindows: Error: [Callstack] 0x00007ffcac24bbe9 UE4Editor-MovieScene.dll!UMovieSceneEntitySystem::Run() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystem.cpp:361] LogWindows: Error: [Callstack] 0x00007ffcac1c65fd UE4Editor-MovieScene.dll!FMovieSceneEntitySystemGraph::ExecutePhase<TArray<unsigned short,TInlineAllocator<8,TSizedDefaultAllocator<32> > > >() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemGraphs.cpp:760] LogWindows: Error: [Callstack] 0x00007ffcac21c264 UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::GameThread_InstantiationPhase() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:371] LogWindows: Error: [Callstack] 0x00007ffcac21dd70 UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::GameThread_SpawnPhase() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:354] LogWindows: Error: [Callstack] 0x00007ffcac21d8d7 UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::GameThread_ProcessQueue() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:272] LogWindows: Error: [Callstack] 0x00007ffcac20bbc6 UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::DoFlushUpdateQueueOnce() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:179] LogWindows: Error: [Callstack] 0x00007ffcac219bfd UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::Flush() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:147] LogWindows: Error: [Callstack] 0x00007ffcac29aeba UE4Editor-MovieScene.dll!FMovieSceneRootEvaluationTemplateInstance::Evaluate() [G:\UE4_27\Engine\Source\Runtime\MovieScene\Private\Evaluation\MovieSceneEvaluationTemplateInstance.cpp:147] LogWindows: Error: [Callstack] 0x00007ffca5745d7e UE4Editor-Sequencer.dll!FSequencer::EvaluateInternal() [G:\UE4_27\Engine\Source\Editor\Sequencer\Private\Sequencer.cpp:3350] LogWindows: Error: [Callstack] 0x00007ffca57d47a9 UE4Editor-Sequencer.dll!FSequencer::Tick() [G:\UE4_27\Engine\Source\Editor\Sequencer\Private\Sequencer.cpp:815] LogWindows: Error: [Callstack] 0x00007ffcb1555449 UE4Editor-UnrealEd.dll!FTickableEditorObject::TickObjects() [G:\UE4_27\Engine\Source\Editor\UnrealEd\Public\TickableEditorObject.h:45] LogWindows: Error: [Callstack] 0x00007ffcb196e318 UE4Editor-UnrealEd.dll!UEditorEngine::Tick() [G:\UE4_27\Engine\Source\Editor\UnrealEd\Private\EditorEngine.cpp:1502] LogWindows: Error: [Callstack] 0x00007ffcb22706e6 UE4Editor-UnrealEd.dll!UUnrealEdEngine::Tick() [G:\UE4_27\Engine\Source\Editor\UnrealEd\Private\UnrealEdEngine.cpp:426] LogWindows: Error: [Callstack] 0x00007ff7ea2a87a0 UE4Editor.exe!FEngineLoop::Tick() [G:\UE4_27\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:4922] LogWindows: Error: [Callstack] 0x00007ff7ea2c0fbc UE4Editor.exe!GuardedMain() [G:\UE4_27\Engine\Source\Runtime\Launch\Private\Launch.cpp:178] LogWindows: Error: [Callstack] 0x00007ff7ea2c10aa UE4Editor.exe!GuardedMainWrapper() [G:\UE4_27\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:137] LogWindows: Error: [Callstack] 0x00007ff7ea2c40cd UE4Editor.exe!LaunchWindowsStartup() [G:\UE4_27\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:273] LogWindows: Error: [Callstack] 0x00007ff7ea2d5984 UE4Editor.exe!WinMain() [G:\UE4_27\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:320] LogWindows: Error: [Callstack] 0x00007ff7ea2d853a UE4Editor.exe!__scrt_common_main_seh() [d:\agent\_work\5\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288] LogWindows: Error: [Callstack] 0x00007ffda4047034 KERNEL32.DLL!UnknownFunction [] LogWindows: Error: [Callstack] 0x00007ffda5a82651 ntdll.dll!UnknownFunction [] LogWindows: Error:
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-131975 in the post.