Webhooks not working from GitLab

Having a similar error between Mattermost and Gitlab. Prior to upgrading to 8.5.1, the integration seemed to be working fine. After upgrading, every time the outgoing hook from Gitlab fires (testing or a real event), it throws up a 500 error. In sidekiq.log I see:

"error_message"=>"getaddrinfo: Name or service not known", "error_class"=>"SocketError"

Followed by:

WARN: SocketError: getaddrinfo: Name or service not known

dig shows that address resolution is working OK, and curl posts out to Mattermost successfully, so I'm not sure what's throwing the error.

EDIT: More information from production.log

Started GET "/testingroup/mattermost/services/slack/test" for 127.0.0.1 at 2016-02-26 11:37:36 -0500
Processing by Projects::ServicesController#test as HTML
Parameters: {"namespace_id"=>"testingroup", "project_id"=>"mattermost", "id"=>"slack"}
Completed 500 Internal Server Error in 137ms (ActiveRecord: 5.6ms)

SocketError (getaddrinfo: Name or service not known):
app/models/project_services/slack_service.rb:101:in execute'
app/models/service.rb:106:intest'
app/controllers/projects/services_controller.rb:49:in `test'