## Context
Upgrading clusters 1.10.0 -> 1.11.x consistently breaks cluster communication with 'h2 protocol error: http2 error: stream error received: frame with invalid size', rendering many operations non-functional.
## Verified resolution
Maintainer timvisee's guidance: with no incoming writes, upgrade each node in isolation (cluster comms not needed), then use the --reinit flag (added in 1.12.2 via PR #5265) on the final upgrade; always snapshot/back up the cluster first. A community member posted a verified step-by-step upgrade path in the thread. Closed as completed.