Which version of DEVONthink & macOS do you use? Does it work again after downloading the app from our website (DEVONtechnologies | Apps) and replacing the installed copy with the downloaded one?
You’re on DT3, I guess? If so, this kind of information is always useful for others.
In any case, you can try accessing the script via the left sidebar:
The extras are hosted on DEVONtechnologies’ servers. You might be unable to connect because of a missing security certificate. A blank Support Assistant window has been a symptom of this in other cases. See here: