I've been getting a similar NIL type error since starting BG (about a week ago) so I presume it has to do with getting honor. Fighting/quests are no problem. When I get a chance I'll grab a SS of the error.
I've submitted this to the author's portal on WoWInterface but thought I'd mention this here as well. It seems with the latest version of SCL, my pet's Claw and Bite abilities are not showing up in the log. My pet's regular melee damage shows up, my pet's Growl ability shows up, and I have confirmed that my pet is casting Claw and Bite correctly as the messages for those abilities show up in the default Blizzard combat log. Also, Claw and Bite show up when the pets of other hunters use those abilities on me. Since I'm 99% sure that Claw and Bite worked before in SCL, and my settings are all the same, did something change in the parser that would've affected them?
The screenshot is from TBC beta, but the error message is indicative of what I'm seeing on the 2.0 live servers. Specifically in the Naxxramas abomination wing near the frogger slimes, dozens of CHAT_MSG_SPELL_* messages are being spammed. According to a guildmate who paired down addons until it disappeared, SCL appears to be the culprit.