- fixed: Actors cannot be linked into the world until after all actors and sectors have been deserialized.

Two issues:

Portal linking requires all skyboxes in the sectors to be present, without them some info is not there when needed.
UpdateWaterLevel was called in AActor::Serialize, which operated on the freshly loaded level where lots of things haven't been set up yet and plane heights may be wrong.

src/p_mobj.cpp

@@ -397,8 +397,6 @@ void AActor::Serialize(FArchive &arc)
 
 	if (arc.IsLoading ())
 	{
-		touching_sectorlist = NULL;
-		LinkToWorld (false, Sector);
 		AddToHash ();
 		SetShade (fillcolor);
 		if (player)
@@ -415,11 +413,16 @@ void AActor::Serialize(FArchive &arc)
 				Speed = GetDefault()->Speed;
 			}
 		}
-		ClearInterpolation();
-		UpdateWaterLevel(false);
 	}
 }
 
+void AActor::PostSerialize()
+{
+	touching_sectorlist = NULL;
+	LinkToWorld(false, Sector);
+	ClearInterpolation();
+	UpdateWaterLevel(false);
+}
 
 AActor::AActor () throw()
 {