---------------------------
dot.exe - Fehler in Anwendung
---------------------------
Die Anweisung in "0x00471fae" verweist auf Speicher in "0x000000e0". Der Vorgang

"read" konnte nicht auf dem Speicher durchgeführt werden.

Klicken Sie auf "OK", um das Programm zu beenden.
Klicken Sie auf "Abbrechen", um das Programm zu debuggen.
---------------------------
OK   Abbrechen
---------------------------

[erg] Looking more closely at DOT's grammar, I realize I was mistaken: your input graph should be legal. I'm guessing it has something to do with cluster_X and cluster_Y being first created as subgraphs of M, and then later being put into cluster_XY. In any case, another workaround is to move the definitions of the subclusters after cluster_XY.

digraph M {
   A -> B
   C -> D
   subgraph cluster_XY { label="XY"; subgraph cluster_X;subgraph cluster_Y; }
   subgraph cluster_X  { label="X"; A;B; }
   subgraph cluster_Y  { label="Y"; C;D; }
}

[erg] Allowing a subgraph with no body currently causes cycles in the subgraph graph, which can cause serious problems, as here. Allowing this in the language is not worth the trouble of implementing all the subtleties correctly. Therefore, the "fix" for this bug is to deprecate and, eventually, disallow non-defining subgraph references. Thus, the appropriate graph is

digraph M {
   A -> B
   C -> D
   subgraph cluster_XY {
     label="XY";
     subgraph cluster_X  { label="X"; A;B; }
     subgraph cluster_Y  { label="Y"; C;D; }
  }
}