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E91 — BACKTRACE_READ_FAILED

Reading the crash backtrace saved in flash failed.

What happens​

The error is logged and read_saved_backtrace returns early — no stacktrace is added to the error log. Boot continues normally. This error appears in two contexts:

  • During verify_panic_flag (release builds, after a double-panic): the backtrace length could not be read from flash. The NVS region may still be partially erased for recovery.
  • During read_saved_backtrace: called after Wi-Fi connects; the backtrace from the previous crash session could not be read from its location in flash.

Possible causes​

  • A flash read error at the backtrace storage location (end of the NVS partition).
  • The backtrace was never written (the crash handler itself failed before saving it).
  • The NVS partition boundary calculation was wrong.

How to fix​

  1. No immediate action needed — The device boots normally. The lost backtrace means the crash that caused the previous panic cannot be symbolised, but the device itself is operational.
  2. Report the crash — If E91 is accompanied by a previous panic (look for a Panic entry in the error log), open an issue on GitHub with your firmware version and a description of what happened before the crash.