Unable to cast ANY spells - "You are already casting a spell! You prepare this spell as a follow up!"

Problem:
I'm unable to cast any spells. Trying to will prompt the "You are already casting a spell! You prepare this spell as a follow up!" message.

Environment:
This happened while I was in a Sidi Raid with many players - a lot of client induced lag was present. I was spamming [Tab] (target next monster) and [1] (LvL 50 Theurg base DD). Then every spell queued, but nothing was cast.

A few things I tested:
-Casting normal spell (Buffs, Base DD, etc): Doesn't work. "You are already casting a spell! You prepare this spell as a follow up!"
-Using Quickcast: Doesn'T work. "You can't prepare a quickcast while casting a spell!"
-Using personal bind: Doesn't work. You are already casting a spell."
-Using a pot: Works.
-Using a Tinderbox: Doesn't work. "You are already casting a spell."
-Using Purge RA: Works.
-Using Instant Debuffs: Works. This one gives correct messages when trying to use it without or with friendly targets, too.

Possible solutions I've tried - none of them worked:
-Death.
-Changing zones.
-Hitting monsters in melee with my staff.
-Unequipping all my items.
-Emotes.
-Sitting.
-Destroying the Personal Bind Stone and using the new one.

Interesting:
Not selecting a target while using my base DD prompts the "You are already casting a spell! You prepare this spell as a follow up!" message, as opposed to the "You mustselect a target!..." one.
Selecting a friendly target gives the same message.