Hello,
You had this error you mentioned + some huge 100s latencies?
We received lots of these errors on October 13th, it lasted for 6 hours straight. We believe it was due to some Datastore slowness.
We opened a google's support case, still, they had no clue what happened and just pointed us at some random documents. We got very frustrated with their answer, I mean, datastore is a managed service, no logs or metrics for us mortals... then today we escalated the case to see if we can get a specialist and some advice on how to handle those errors in the future.
Have you realized as well that this error and the 100s timeout is not even documented?
geez