While sending out a translation notification on Meta, after hitting the "send notification to translators" button and waiting for a bit, I received this error page:
504 Gateway Time-out nginx/1.1.19
The notification itself seems to have gone out fine, here's the log entry:
05:16, 10 February 2014 Tbayer (WMF) (talk | contribs) sent a notification about translating page Data retention guidelines; languages: all languages; deadline: none; priority: high; sent to 1469 recipients, failed for 0 recipients, skipped for 6 recipients
This bug continues the hallowed tradition of T43131: Timeout when sending translation notification and T57397: Timeout when sending translation notification (again). Both of which have been fixed and had different kinds of error messages (not 504), which is why I'm filing a new bug rather than reopening one of them.