The GAE redeploys each time this happens. Some days it happens more than others, but today it has happened every hour
Update: it is now happening every 30 mins. This error is causing multiple new instances to run creating unnecessary billing. I can't even contact Billing Support because This organisation isn't eligible to file tech-related support cases. To do so, we invite you to upgrade your service package.
Region is us-1-east
Deleting all previous App Engine versions and all unused images in Container Registry under
app-engine-tmp/app/default/.../did the trick. I don't know whether it's one or the other (or both) actions that fixed the error, but it hasn't happened again since then