Refresh/sign-in fails with NSCocoaErrorDomain 3840; HTTP 503 observed during reproduction

AltStore intermittently failed to refresh apps, sometimes succeeding between failures. It now fails to sign in after signing out.

Error:
NSCocoaErrorDomain 3840
“The data couldn’t be read because it isn’t in the correct format.”

Environment and tests:

• AltServer 1.7.2 (build 90) on macOS.
• Reproduced using both a MacBook Pro and a Mac mini.
• Failed over Wi-Fi and with the iPhone connected by USB.
• Restarting AltServer and signing out/back into AltStore did not resolve it.
• Signing into Apple’s developer website succeeds.
• AltStore on iPhone: 2.2.1.
• iOS 27 public beta 6.

Captured evidence:
During one failed sign-in attempt on September 4, 2026, iPhone Console logs attributed to the AltStore process showed:

• A successful TLS handshake on Connection 22.
• Two HTTP 200 responses, followed by an HTTP 503 response on that same connection.
• The 503 request completed in approximately 55 ms, with 477 response bytes reported.

The capture does not identify the endpoint or expose the response body, and it did not contain the 3840 exception itself. Therefore, the 503 is correlated with the failed attempt, but we cannot prove it caused the parsing error or identify which service returned it.

Could you advise how to safely identify the failing endpoint and capture the underlying parsing error without exposing credentials or authentication tokens?

Potentially related: https://github.com/altstoreio/AltStore/issues/1747

Start by reviewing related issue #1747 and reproducing the sign-in failure while examining iPhone Console logs for the AltStore process. No source file or test is named; done means safely identifying the failing endpoint and capturing the underlying parsing error without exposing credentials or authentication tokens.