When the server keeps rejecting this device's /keys/upload with 400 "One
time key ... already exists", the device's encryption store has diverged
and the SDK retries forever (matrix-rust-sdk#5200, still open). Synapse
saw 474k of these from May 30 to Jul 8 2026. The fix is a fresh login, so:
- keyUploadWatch observes /keys/upload responses through the client's
fetchFn (the response the SDK gets is untouched; a clone is read).
Three conflicts within 15 min = stuck; any successful upload clears it.
- KeyUploadBanner (critical strip, same slot as the sync/clock banners):
"Encryption on this device is out of sync" with Details and
"Fix: sign out…", which opens the normal logout dialog (it already
warns when the device isn't verified / backed up).
Server side, Grafana now alerts on the same signal (E2EE Key Upload
Conflicts) and on delayed-event 5xx (Call Delayed-Event Errors).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5
