I've been able to replicate this consistently about 10-15 times. Whenever I cast a spell that's interruptible by movement like Blizzard I or II, or Fire I (dont have access to more on my THU) when I'm already moving and I don't stop moving until the spell is 'finished casting', I get removed from the server with an Error 90000. Sometimes this happens after the casting bar displays the 'interrupted' information, sometimes due to what I presume to be lag, I get booted from the server before the message even appears.

Not entirely sure to what extent it affects casting spells where you do stop moving before the spell was supposed to finish or start moving after you've started casting, but when I'm casting spells and constantly moving it's already annoying enough to have made me reroll a Lancer.