[MrXIII]

The phenomenon you are describing is called skill lock, or Action Slot jamming, I believe, and has been present in this game for a very long while (I experienced it back in V7 around February 2007 for the first time I believe). Since it blocks (used to block?) both the action slot and the F-keys, it renders you skill-less for a moment. The solution really is to simply walk around for a bit, perhaps even jumping would help (i.e. anything that resets your character's position), and after that both F-keys and the action slot should work again.

Also, it has nothing to do with the Heal spell itself, it is the way you use the action slot with it. Repeatedly using the action slot too quickly multiple times can cause this to happen. It has happened in the past on my Mage while spamming Mental Strike via the Action Slot as well, and I believe AoErs who chain their AoE in the Action Slot (e.g. a Knight using Powerstump, or a Psykeeper using Maximum Crisis, or even an Elementor chaining 3 Windfields) are vulnerable to this as well.

So don't refrain from using Heal in the Action Slot. A double- or triple-chained action slot'ed Heal is a very powerful technique, if used sparely. It's just not feasible to spam it though.