27,136 errors in the last 7 days. 20k in the last day.
https://logstash.wikimedia.org/goto/aab8b3e7e699316f3bb04cae5721b99c

Per https://wikitech.wikimedia.org/wiki/Deployments/Holding_the_train#Issues_that_hold_the_train

This meets the criteria of
"Newly introduced bugs that show up on the mw-client-errors dashboard or mw-client-error editing dashboard at a rate of: Over 100 errors in a 1 hour period, Over 1000 errors in a 12 hr period"

Stack traces all point to

at SafeStorage.prototype.getExpiryKeys/

So presumably this relates to T121646