I did have several Enchantments up. I was not aware that your essence determined enchantment limit. Since I usually increase essence when I level up I thought the limit was tied to level. I'm not sure if casting the Imbue would have taken me below the limit because I don't know how much essence is needed per enchantment to maintain them.
Also later in that game I was able to Imbue heroes again but then after Imbuing two it would crash again when I tried to Imbue another. Didn't seem to matter what hero I tried it on it would crash. I did seem like it was tied to some lower limit on my Mana/Essence total. So the enchantment thing could of been it since I had several summons.
By the way how much Essence is need per enchantment slot?