Array out of bounds engine crash in Animation Code on Multiplayer only

Development Character & Animation
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1

The code below crashes my editor, when it is run on a multiplayer setup.

Works perfectly on single player, but on multiplayer it crashes the engine with this error every time.

Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\Epic Games\UE_4.20\Engine\Source\Runtime\Core\Public\Containers/Array.h] [Line: 611]  Array index out of bounds: 139 from an array of size 78

This Section in particular if I comment it out the crashes stop.

// Apply rotation to IK root bone. 
FTransform& IKRootBoneTransform = Output.Pose[IKFootRootBoneCompactPoseIndex]; 
IKRootBoneTransform.ConcatenateRotation(DeltaQuat); 
IKRootBoneTransform.NormalizeRotation();

Main code:

     if (!FMath::IsNearlyZero(LocomotionAngle, KINDA_SMALL_NUMBER)) 	
     { 		
     const FBoneContainer& BoneContainer = Output.AnimInstanceProxy->GetRequiredBones();
    
              // Get bone indexes and verify they exist. 		
     FCompactPoseBoneIndex IKFootRootBoneCompactPoseIndex =           
 IKFootRootBone.GetCompactPoseIndex(BoneContainer);
     FCompactPoseBoneIndex PelvisBoneCompactPoseIndex = 
 PelvisBone.GetCompactPoseIndex(BoneContainer); 		
     if (IKFootRootBoneCompactPoseIndex == INDEX_NONE || 
 PelvisBoneCompactPoseIndex ==INDEX_NONE)
     return;
    
    		 // Prepare convert Quat and
     BoneContainer.FComponentSpacePoseContext CSOutput(Output.AnimInstanceProxy);
     CSOutput.Pose.InitPose(Output.Pose);
    
    		// Build our desired rotations for IK root bone and body. 		
     FRotator DeltaRotation(EForceInit::ForceInitToZero);
     FRotator BodyDeltaRotation(EForceInit::ForceInitToZero);
    
     const float BodyAlpha = FMath::Clamp((Settings.BodyOrientationAlpha - 1.0f), -1.f, 0.f);
     switch (Settings.YawRotationAxis) 		
     { 		
     case EAxis::X: 			
     DeltaRotation.Roll = LocomotionAngle;
     BodyDeltaRotation.Roll = LocomotionAngle * BodyAlpha; 			
     break;
     case EAxis::Y:
     DeltaRotation.Pitch = LocomotionAngle; 
     BodyDeltaRotation.Pitch = LocomotionAngle * BodyAlpha; 
     break;
     case EAxis::Z:
     DeltaRotation.Yaw = LocomotionAngle;
     BodyDeltaRotation.Yaw = LocomotionAngle * BodyAlpha;
     break;
     default:
     break;
     } 
    const FQuat
    DeltaQuat(DeltaRotation);
    
    // Apply rotation to IK root bone. 	
FTransform& IKRootBoneTransform = Output.Pose[IKFootRootBoneCompactPoseIndex]; 
IKRootBoneTransform.ConcatenateRotation(DeltaQuat); 	
IKRootBoneTransform.NormalizeRotation();