Investigate spike in 5xx errors from router-frontend-PROD

Over the last couple of days we've seen a number of [spikes in 5xx errors from the router-frontend-PROD service](https://metrics.gutools.co.uk/d/zM0ouzs4z/availability-dashboard?orgId=1&var-Period=5m&var-AWSAccount=000000101&var-StageFilter=PROD&var-ElkIndexFilter=logstash-frontend-*&var-ElasticSearchIndex=W7i1CLSVk&var-Service=frontend-PROD-router-ELB&var-App=router&var-Stack=frontend&var-Stage=PROD&var-ASGName=frontend-PROD-RouterStack-G1EEL7AIYWIU-AutoscalingGroup-1DO4VJI0GY7JK&var-SLO=0.995&var-RiffRaffProject=dotcom%3Arouter&var-LogLevel=ERROR&var-KibanaErrorsURL=https%3A%2F%2Flogs.gutools.co.uk%2Fs%2Fdotcom%2Fapp%2Fdiscover%23%2F%3F_g%3D%28filters%3A%21%28%28query%3A%28match_phrase%3A%28app%3Arouter%29%29%29%2C%28query%3A%28match_phrase%3A%28stage%3APROD%29%29%29%2C%28query%3A%28match_phrase%3A%28level%3AERROR%29%29%29%29%2CrefreshInterval%3A%28pause%3A%21t%2Cvalue%3A0%29%2Ctime%3A%28from%3A%272025-05-28T09%3A46%3A50.890Z%27%2Cto%3A%272025-05-29T09%3A46%3A50.890Z%27%29%29&var-KibanaWarnURL=https%3A%2F%2Flogs.gutools.co.uk%2Fs%2Fdotcom%2Fapp%2Fdiscover%23%2F%3F_g%3D%28filters%3A%21%28%28query%3A%28match_phrase%3A%28app%3Arouter%29%29%29%2C%28query%3A%28match_phrase%3A%28stage%3APROD%29%29%29%2C%28query%3A%28match_phrase%3A%28level%3AWARN%29%29%29%29%2CrefreshInterval%3A%28pause%3A%21t%2Cvalue%3A0%29%2Ctime%3A%28from%3A%272025-05-28T09%3A46%3A50.890Z%27%2Cto%3A%272025-05-29T09%3A46%3A50.890Z%27%29%29&var-KibanaInfoURL=https%3A%2F%2Flogs.gutools.co.uk%2Fs%2Fdotcom%2Fapp%2Fdiscover%23%2F%3F_g%3D%28filters%3A%21%28%28query%3A%28match_phrase%3A%28app%3Arouter%29%29%29%2C%28query%3A%28match_phrase%3A%28stage%3APROD%29%29%29%2C%28query%3A%28match_phrase%3A%28level%3AINFO%29%29%29%29%2CrefreshInterval%3A%28pause%3A%21t%2Cvalue%3A0%29%2Ctime%3A%28from%3A%272025-05-28T09%3A46%3A50.890Z%27%2Cto%3A%272025-05-29T09%3A46%3A50.890Z%27%29%29&var-KibanaAppFilter=All&var-ASGNameFilter=%28%22AutoScalingGroupName%22%3D%22frontend-PROD-RouterStack-G1EEL7AIYWIU-AutoscalingGroup-1DO4VJI0GY7JK%22%29&from=1747915166307&to=1748519966307). In some cases these coincide with an overall increase in traffic. We should investigate to determine the underlying cause and whether we need to take any action to mitigate further errors.