I seem to be having a very similar problem. I get on initial load the same DrDamage.lua:769: bad argument #1 to 'find' (string expected, to nil) error. If I switch out the helm and reload ui, DrDamage loads error free.
Sayrox may be onto something with the meta gem. The helm in question that gives the error is the Thunderheart Headguard (Moonkin Druid piece), gemmed with a Potent Pyrestone and a Chaotic Skyfire Diamond. If I put on the S3 Vengeful Gladiators Wyrmhide helm with a Swift Starfire Diamond...I go error free.
Hope this helps ;)