I think it is the same issue that was fixed in version 2.6 that has recurred.
https://tracker.moodle.org/browse/MDL-43314

This time I'm on a Moodle 2.7.1 (Build: 20140731)
Windows 2008 R2 Servers with php 5.5.12 and MySQL 5.6.1
The cron is set up as a scheduled task on a separate server to the main Moodle.
It just stopped functioning, no plugins added, no themes that I'm aware of. We thought it was working because the RSS feeds were updating, but forum messages were not being sent.

This was not a fresh install, it was a 2.4 upgraded twice.
We do not install automatic updates, we just have it notify us when there are some.