Google calendar integration stopped working

I woke up this morning and found someone has tried to make a booking for January 4th, however we are already fully booked for January 4th. Upon checking I now have no appointments reserved by Google calendar.

I tested my calendar integration and it returns an error, looking at the logs I can see:

[29 Dec 07:32] Error fetching Google events: 6
[29 Dec 07:44] Error fetching Google events: 6
[29 Dec 08:07] Error fetching Google events: 6
[29 Dec 08:17] Error fetching Google events: 6
[29 Dec 08:27] Error fetching Google events: 6
[29 Dec 08:37] Error fetching Google events: 6
[29 Dec 08:51] Error fetching Google events: 6

Everything was working fine up until now, I've had this configured with no problems for over a year and I've changed nothing. The problem only started happening 1.5 hours ago judging by the logs.

Any ideas? Have Google changed something on the API?