Solved

Training finction isn't working. Please unlock

Clear cache and it will work, Google Chrome?

Still happening? I don't see it.

It's basically random, we're not even touching it! The server is just deciding to do this or it doesn't.

We're setting up a new system for next week, with all the benefits of this one but also a much better ability to trace any unusual behaviours from it. As part of moving everything to the cloud we also migrated from Linux to Windows, but now we're going to move back to Linux. It's a system we understand better and most of these unsusual behaviours seem to come from the unique ways PHP and IIS interact.

If this is all Greek to you, we're going back to the drawing board. :P