Removing D_user check from AddWindowFlags() prevents the crash. I hope it doesn't have any serious consequences with a different setup.

diff --git a/src/process.c b/src/process.c
index e494004..a18bc28 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5634,11 +5634,7 @@ struct win *p;
 #endif
   if (p->w_layer.l_cvlist && p->w_layer.l_cvlist->c_lnext)
     *s++ = '&';
-  if (p->w_monitor == MON_DONE
-#ifdef MULTIUSER
-      && (ACLBYTE(p->w_mon_notify, D_user->u_id) & ACLBIT(D_user->u_id))
-#endif
-     )
+  if (p->w_monitor == MON_DONE)
     *s++ = '@';
   if (p->w_bell == BELL_DONE)
     *s++ = '!';

I missed the trigger: defmonitor on
Now I tested vanilla 2f565ea with config:
defmonitor on
bind x screen -t G group
bind o windowlist  

Steps to reproduce:
1) create a group inside screen session: screen -t G group
2) enter this group, create some windows inside and open windowlist
3) attach to this screen session from another term: screen -x
4) go to group G and try to create new window or rename existing one

Then, there is either a dead screen session or crash:
Suddenly the dungeon collapses!! - You die...

5) the effect is the same if autoaka changes title

A punishment for all cheating in Nethack!