AppleScript referencing tabs suddently breaking on Chrome
I can actually get your example to work with no problems, just doing something simple:

tell application "Google Chrome"
  tell window 1
    tell tab 1
        get URL
    end tell
  end tell
end tell


That makes me think the problem might be in the "do something here" section of your script—I've never found Applescript's error messages to be very reliable.

That said, you could also try shortening your code:

tell application "Google Chrome"
  tell active tab of first window
    [do something here]
  end tell
end tell


Using my earlier, super simple example, this works for me:

tell application "Google Chrome"
  get URL of active tab of first window
end tell


I should also note that I'm using Script Editor 2.7 (176) on Yosemite 10.10.1