We're aware the build service has had some issues with reliability in the past few weeks. We apologise for the inconvenience and we're keen to identify the problem and ensure it is fixed.
Unfortunately the type of problem is very difficult to diagnose - as you may know from our bug report guidelines, it's important to have reliable steps to reproduce a problem in order to investigate it. We do not have any steps to reproduce the issue with the build service, and the problem appears to happen unpredictably after long periods of time, often taking several days or weeks before it occurs. Investigating a problem often involves trying dozens of potential solutions. Combined with the long duration before the problem occurs, this means investigating the problem may be a long term process.
Nonetheless we're aware of the importance of the build service to many customers and so we will be taking steps to diagnose and resolve the problem. Part of this will be running diagnostics and maintenance on the build server. We are planning to do some work Wednesday December 1st (tomorrow) from 11:00 to 18:00 UTC. The build service may be fully unavailable, or only partially available, during this time. Once again we apologise for the inconvenience, but this work is likely a necessary step to ensuring the reliability of the build service.
In the mean time if you need an alternative to the build service, you can run your own Cordova builds locally with the Cordova CLI, as documented in the tutorial Building mobile apps locally with Cordova CLI. Thanks for your patience.