According to its text, Cipher Ex Dragon should be able to be summoned by using any "Cipher Dragon" monster as material, whether they are Xyz monsters or not, but when a monster is taken and turned into a Cipher Dragon via the original Galaxy-Eyes Cipher Dragon, it isn't read as a suitable material for Ex Dragon.
Works fine for me.

Can you supply a replay stating when you tried to Special Summon the monster in question and were unable too?