Recovery Key Confirmation Doesn't Work Properly

[matrixbot]

This issue was originally created by @shimi-ke at matrix-org/dendrite#3329.

Background information

• Dendrite version or git SHA: 0.13.6
• SQLite3 or Postgres?: Postgres
• Running in Docker?: no
• go version: 1.20.8
• Client used (if applicable): Element X iOS 1.5.7

Description

• What is the problem: I cannot confirm my recovery key from Element X app.
• How is this bug manifesting: If I type my recovery key (which I double-checked in Element Web) I get an error message.
• When did this first appear: From the first version of the Element X app which implemented recovery key confirmation.

Steps to reproduce

• I enter the recovery key in the client app
• The first log entry in access log in reverse proxy running in front of the Dendrite server (Traefik v2.11) is:
  GET /_matrix/client/v3/user/<redacted>/account_data/m.secret_storage.default_key HTTP/1.1 ends with 200 status code
• The second log entry might be the core of the issue:
  GET /_matrix/client/v3/user/<redacted>/account_data/m.org.matrix.custom.backup_disabled HTTP/1.1 which ends with 404 status code
• The last related log entry is:
  POST /_matrix/client/v3/keys/signatures/upload HTTP/1.1 which ends with 200 status code

P.S.
I have reported this issue as a bug report from within the Element X as well.