I don’t recall seeing that exception when we had problems with redis, but it may be my memory failing.
We also used elasticache and we faced the problem of memory filling up. You can see a discussion on this PR: fix: allow to setup redis maxmemory by Ian2012 · Pull Request #984 · overhangio/tutor · GitHub. TLDR: the course structure cache was not being evicted unless you added some extra configuration (this didn’t happen before because the behavior of memcache is different)
For elasticache what we ended up doing was to set maxmemory-policy to allkeys-lru from the default volatile-lru.