I tried to update the addon myself several days ago and ran into that UnitGUID bug too. I managed to get rid of the lua errors by checking for nil around line 687 in MSBTParser.lua. I haven't noticed any glaring bugs come out of it, but at least it got the error spam to stop.
Code:
if(guid ~= nil and unitMap ~= nil) then
unitMap[guid] = unitID
if (not classMap[guid]) then _, classMap[guid] = UnitClass(unitID) end
classTimes[guid] = nil
end