On the morning of May 1, 2020, XTools was returning a 504 Gateway Timeout error on and off for about three hours.
According to our uptime monitoring, the service was down for about 3 hours. I was not able to identify any reason for this. There have been some bots hammering the service lately, but this shouldn't cause complete downtime, rather just the "service is overloaded" error, and I didn't get any automated emails about that as I would normally. No other VPS instances I maintain went down during this time, so I am not led to believe it was an issue with Cloud Services.
I'm going to leave this open for the time being. Hopefully whatever it was fixed itself...
It's been several months since this incident. I still don't know what happened, but XTools has received no downtime since then. Whatever the issue was, it apparently has resolved itself. Closing.