As far as we can tell, this applies to Flow and only to Flow.
Attempt to save a topic at http://en.wikipedia.beta.wmflabs.org/wiki/Talk:Flow_QA. Result is a red error "An error occurred while contacting the server." and js console shows a 503.
In the logs we find:
[proxy_fcgi:error] [pid 19052] (70014)End of file found: [client 10.68.16.16:4302] AH01075: Error dispatching request to :, referer: http://en.wikipedia.beta.wmflabs.org/wiki/Talk:Flow_QA
No services other than Flow seem to be affected.
