The only interesting clue I see in the log files is below. It says "User excluded" which is surprising because the user in question is root. To test if there really is a user permission problem, I added root as an Owner of the group, and repeated the test, with still the same error. I also repeated as myself (I'm an Administrator, and an Owner of the group), still the same result.
Started POST "/api/v3/projects/somegroup%2Fnewproject/hooks" for 127.0.0.1 at 2016-05-19 15:48:16 -0400 ** [Raven] User excluded error: #<ActionController::RoutingError: No route matches [POST] "/api/v3/projects/somegroup%2Fnewproject/hooks"> ActionController::RoutingError (No route matches [POST] "/api/v3/projects/somegroup%2Fnewproject/hooks"): actionpack (4.2.6) lib/action_dispatch/middleware/debug_exceptions.rb:21:in `call' actionpack (4.2.6) lib/action_dispatch/middleware/show_exceptions.rb:30:in `call' railties (4.2.6) lib/rails/rack/logger.rb:38:in `call_app' railties (4.2.6) lib/rails/rack/logger.rb:20:in `block in call' activesupport (4.2.6) lib/active_support/tagged_logging.rb:68:in `block in tagged' activesupport (4.2.6) lib/active_support/tagged_logging.rb:26:in `tagged' activesupport (4.2.6) lib/active_support/tagged_logging.rb:68:in `tagged' railties (4.2.6) lib/rails/rack/logger.rb:20:in `call' request_store (1.3.0) lib/request_store/middleware.rb:9:in `call' actionpack (4.2.6) lib/action_dispatch/middleware/request_id.rb:21:in `call' rack (1.6.4) lib/rack/methodoverride.rb:22:in `call' rack (1.6.4) lib/rack/runtime.rb:18:in `call' rack (1.6.4) lib/rack/lock.rb:17:in `call' rack (1.6.4) lib/rack/sendfile.rb:113:in `call' sentry-raven (0.15.6) lib/raven/integrations/rack.rb:53:in `call' railties (4.2.6) lib/rails/engine.rb:518:in `call' railties (4.2.6) lib/rails/application.rb:165:in `call' railties (4.2.6) lib/rails/railtie.rb:194:in `public_send' railties (4.2.6) lib/rails/railtie.rb:194:in `method_missing' rack (1.6.4) lib/rack/urlmap.rb:66:in `block in call' rack (1.6.4) lib/rack/urlmap.rb:50:in `each' rack (1.6.4) lib/rack/urlmap.rb:50:in `call' unicorn (4.9.0) lib/unicorn/http_server.rb:580:in `process_client' unicorn-worker-killer (0.4.4) lib/unicorn/worker_killer.rb:52:in `process_client' unicorn (4.9.0) lib/unicorn/http_server.rb:674:in `worker_loop' unicorn (4.9.0) lib/unicorn/http_server.rb:529:in `spawn_missing_workers' unicorn (4.9.0) lib/unicorn/http_server.rb:540:in `maintain_worker_count' unicorn (4.9.0) lib/unicorn/http_server.rb:294:in `join' unicorn (4.9.0) bin/unicorn:126:in `<top (required)>' /opt/gitlab/embedded/service/gem/ruby/2.1.0/bin/unicorn:23:in `load' /opt/gitlab/embedded/service/gem/ruby/2.1.0/bin/unicorn:23:in `<main>' -- You received this message because you are subscribed to the Google Groups "GitLab" group. To unsubscribe from this group and stop receiving emails from it, send an email to gitlabhq+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/gitlabhq/BY1PR0401MB125596DF9EFDC90FCDEB7CB9DC4A0%40BY1PR0401MB1255.namprd04.prod.outlook.com. For more options, visit https://groups.google.com/d/optout.