Plug.Conn.__struct__/0 is undefined

:appsignal_plug, a dependency of :appsignal_phoenix, fails on this line, with an error Plug.Conn.__struct__/0 is undefined.

Any idea on what might be causing this, and how to fix it, appreciated.

What I’ve tried so far:

• rm -rf _build deps
• mv ~/.hex/cache.ets ~/.hex/cache.ets.bak
• locally cloned and successfully compiled :appsignal_plug ( {:appsignal_plug, path: "/appsignal-elixir-plug", override: true})
• removed :appsignal_* deps, but now :bandit reports same error (module Plug.Conn is not loaded and could not be found).

Full error from mix deps.compile output:

==> appsignal
AppSignal extension installation successful
Compiling 54 files (.ex)
Generated appsignal app
==> appsignal_plug
Compiling 2 files (.ex)
     error: Plug.Conn.__struct__/0 is undefined, cannot expand struct Plug.Conn. Make sure the struct name is correct. If the struct name exists and is correct but it still cannot be found, you likely have cyclic module usage in your code
     │
 100 │   def put_name(%Plug.Conn{} = conn, name) do
     │                ^
     │
     └─ lib/appsignal_plug.ex:100:16: Appsignal.Plug.put_name/2

    error: Plug.Conn.__struct__/0 is undefined, cannot expand struct Plug.Conn. Make sure the struct name is correct. If the struct name exists and is correct but it still cannot be found, you likely have cyclic module usage in your code
    │
  3 │         %Plug.Conn{
    │         ^
    │
    └─ lib/appsignal_plug/metadata.ex:3:9: Appsignal.Metadata.Plug.Conn.metadata/1


== Compilation error in file lib/appsignal_plug.ex ==
** (CompileError) lib/appsignal_plug.ex: cannot compile module Appsignal.Plug (errors have been logged)
    lib/appsignal_plug.ex:100: (module)
could not compile dependency :appsignal_plug, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile appsignal_plug --force", update it with "mix deps.update appsignal_plug" or clean it with "mix deps.clean appsignal_plug"

Odd things noticed:

• the above error occurs even when referring to a locally-cloned :appsignal_plug ({:appsignal_plug, path: "/appsignal-elixir-plug"}) … even though that locally-cloned repo compiles!
• :appsignal_plug compiles in another, older instance of my application, on the same server. The problematic application’s mix.lock is identical to the older one’s, regarding the 3 AppSignal libs. Both mix.locks have this:

  "appsignal": {:hex, :appsignal, "2.12.2", "f93bed975c73779e1711fb2196b4c340c50b474ab41a7026e8599b0b4b5db046", [:make, :mix], [{:decimal, "~> 2.0", [hex: :decimal, repo: "hexpm", optional: false]}, {:decorator, "~> 1.2.3 or ~> 1.3", [hex: :decorator, repo: "hexpm", optional: false]}, {:hackney, "~> 1.6", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "c4063fcd264a5e372299bec15bbf35a057ae5912c739d374616e0a33c0cc9ef0"},
  "appsignal_phoenix": {:hex, :appsignal_phoenix, "2.4.0", "150cf8753e3f0a6352504886808c822a4b8099afdfb4b8ff98cbd8d49c99663d", [:mix], [{:appsignal, ">= 2.11.0 and < 3.0.0", [hex: :appsignal, repo: "hexpm", optional: false]}, {:appsignal_plug, ">= 2.0.15 and < 3.0.0", [hex: :appsignal_plug, repo: "hexpm", optional: false]}, {:hackney, "~> 1.6", [hex: :hackney, repo: "hexpm", optional: false]}, {:phoenix, "~> 1.4", [hex: :phoenix, repo: "hexpm", optional: false]}, {:phoenix_html, "~> 2.11 or ~> 3.0 or ~> 4.0", [hex: :phoenix_html, repo: "hexpm", optional: true]}, {:phoenix_live_view, "~> 0.9 or ~> 1.0", [hex: :phoenix_live_view, repo: "hexpm", optional: true]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "2808cbf12c0ab16ae909ccbae1af758d7478e6165a4324277f701bafbcf70aa1"},
  "appsignal_plug": {:hex, :appsignal_plug, "2.0.15", "758a8a78944878e8461bbc77ca86219121a56f4299c6d79940ab083cf9afea00", [:mix], [{:appsignal, ">= 2.7.6 and < 3.0.0", [hex: :appsignal, repo: "hexpm", optional: false]}, {:plug, ">= 1.1.0", [hex: :plug, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "1c6059049e2081e808aaef04e2b9917e06277f61a35a0e103db860d08cbc41f1"}

• I checked the 2 myapp versions for any difference in Plug usage, but grep Plug /myapp{newer,older}/lib/**/*.ex output is identical.

Env

Elixir 1.17.1
Erlang 27.0
Ubuntu 20.04.6