Start translation for languages for which Apertium option is present - e.g. es to ca. The following error will be displayed:
http://cxserver2.wmflabs.org/v2/translate/es/ca/ 500 (Internal Server Error) -- Response for preflight is invalid (redirect)
This issue seems to only happen on the testing servers (example), and has no visible effect for users apart from the error being logged in the browser console. It may be related to the access through http: (instead of https:) to the testing servers.