"Unknown error" connecting to GitLab

[kussmaul]

I just added this extension (to use with a Google Doc add-on I wrote several years ago). I can connect to GitHub without any problems, push my code to a new repo, etc.
When I try to connect to GitLab, I can login, but when I open the Script Editor I get a red dialog with "Unknown Error", and none of the controls (Repository, Branch, etc) do anything.