- 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/g_level.cpp

@@ -1580,6 +1580,13 @@ void G_SerializeLevel (FArchive &arc, bool hubLoad)
 	P_SerializeSounds (arc);
 	if (arc.IsLoading())
 	{
+		FThinkerIterator it(RUNTIME_CLASS(DThinker));
+		DThinker *th;
+		while ((th = it.Next()))
+		{
+			th->PostSerialize();
+		}
+
 		for (i = 0; i < numsectors; i++)
 		{
 			P_Recalculate3DFloors(&sectors[i]);