Odd as it may sound, this is almost certainly not a graphics issue, and also not a problem of the game per se - this is very likely a connection stability problem in disguise.
More specifically: the mentioned spell includes a movement-affecting component, and if the player character is out of sync with the game server said movement cannot be processed anymore. This triggers a disconnection, as the server cannot move the character where they actually should be - and to get out of sync in that fashion it almost always comes down to undue losses/delays of data packets between computer and realm.
If the Mac is currently connected via Airport/WLAN (which is notoriously vulnerable to delay/losses in data packet transmission), swapping to an ethernet cable connection instead may be all it takes to get things back on track. Beyond that, it may be worth a shot to try a VPN to change the external connection route - similar issues could occur if any of the network nodes the internet company routes the signal through is troublesome, and a VPN may allow to bypass said nodes.