What version are you running?
3.0.1
What's the URL of the page containing the problem?
https://reviewboard.example.com/admin/db/scmtools/repository/add/
What steps will reproduce the problem?

Login to reviewboard as an administrator
Click your username in the upper right corner
Click "Admin"
Click "Add" on the repositories line on the left of the screen in the Manage block
Fill in the name field
Choose gitlab as Hosting service
Select "<Link a new account>" for Account
Put service URL on "https://gitlab.com"
Fill in a username and password of a valid gitlab.com account
Fill in a repository name in this gitlab account
11 Click save

What is the expected output? What do you see instead?
Have the page close and the repository be added. Now I get:
"Unknown error when linking the account: A GitLab server was not found at the provided URL.. The details of the failure are in the Review Board log file."
What operating system are you using? What browser?
Ubuntu 14.04 with Google Chrome 62
Please provide any additional information below.
Reviewboard makes use of a removed feature of Gitlab. The "/api/v4/session" endpoint is used in: https://github.com/reviewboard/reviewboard/blob/c3b5f2d2f94f72caad7f7d23646589aa8a140c1e/reviewboard/hostingsvcs/gitlab.py#L242 But it was removed in Gitlab version 10.2.0. See changelog https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md#1020-2017-11-22. If your gitlab instance was already connected the Hosting service will continue to operate. There also is a way around the problem by generating an access token by hand in gitlab interface (this is currently not possible through the API) and encypting it and adding the hosting account directly under "Hosting Accounts"
The code I used to encrypt my token and add it by hand is below. A screenshot with the apropiate json structure is attached. The secret can be found in your

settings.local.py file.

import sys
from reviewboard.scmtools.crypto_utils import (decrypt_password,
                                                       encrypt_password)
non_encrypt_token             = sys.argv[1]
first_16_characters_of_secret = sys.argv[2]

encrypted = encrypt_password(non_encrypt_token, first_16_characters_of_secret)

print encrypted


Since gitlab has no option to add "access tokens" using the API (at least at the time of writing), I think the best approach whould be te remove the username/password fields, and replace them with an "Access Token" field. If need more information, please feel free to contact me.