Packaged: Assertion Failed: IsInGameThread() || IsAsyncLoading() [MaterialInstance.cpp Line: 809]

My game runs fine simulated, PIE, standalone, but when I start the packaged version, I get an error message saying

Packaged: Assertion Failed: IsInGameThread() || IsAsyncLoading() [MaterialInstance.cpp Line: 809]

So some problem with some material instance somewhere.

That’s the code in the source file that has the asserts:

const UMaterial* UMaterialInstance::GetMaterial() const
{
	check(IsInGameThread() || IsAsyncLoading());
	if(GetReentrantFlag())
	{
		return UMaterial::GetDefaultMaterial(MD_Surface);
	}

	FMICReentranceGuard	Guard(this);
	if(Parent)
	{
		return Parent->GetMaterial();
	}
	else
	{
		return UMaterial::GetDefaultMaterial(MD_Surface);
	}
}

I don’t use any TextRender in my project.