Answer
Check for oversized cookies being added to requests by something other than IFS itself (load-balancer or network-identity cookies are common culprits) — capturing a HAR file when the error occurs will show which cookies are unusually large. Combined with that, confirm the environment's ingress is configured with a large-enough header buffer (older setups default to 8k, while a fix bumping it to 16k needs both a sufficiently recent platform SU and Helm version, and older on-prem/remote infrastructure sometimes doesn't get that increase fully applied even after upgrading) — check the ingress config directly for the header-buffer setting if this keeps recurring.