From an app on Private Spaces, I see H99 - Platform errors in the app logs. Visitors to the app reports they occasionally get "400 Bad Request" responses.
Migrate the app to the upgraded routing infrastructure for Private Spaces. Please review the Dev Center article Routing in Private Spaces for the detailed procedure.
Heroku upgraded the routing infrastructure for Private Spaces on October 26, 2016. The upgraded routing infrastructure supports higher request throughput and reduced latency. Private Space applications created before this date is on the original routing infrastructure if the following command shows
Via: 1.1 vegur while an app on the upgraded routing infrastructure shows
Via: 1.1 dogwood-router or
Via: 1.1 spaces-router followed with an internal version number.
curl -s -I http://<app-name>.herokuapp.com/ | grep Via:
The upgraded routing infrastructure provides improvements in the areas including more accurate error code reporting and larger cookie and header support. The original routing infrastructure in Private Spaces was reporting H99 errors in place for more specific H errors. Please refer to the Dev Center article Private Space Routing Infrastructure for the details about the upgraded routing infrastructure.