[E00007] Sandbox credentials become invalid

[mmedvedev2]

Environment: Sandbox (apitest.authorize.net)
Error: [E00007] User authentication failed due to invalid authentication values

Our team has different sandbox accounts (these are not shared — every person and every dev server uses its own API Login ID / Transaction Key). At some point all of these independent accounts started returning E00007 at roughly the same time.

Symptoms (per account)
- When we generate a new API Login ID / Transaction Key in the Merchant Interface, the credentials work for a while but only for several hours.
- After that they start returning E00007 again, without us changing anything on our side.
- Regenerating the Transaction Key fixes it temporarily, then it breaks again — consistently, no matter how many times we regenerate.
- Right after regeneration a test transaction succeeds, so it is not a typo or a copy/paste issue.
- Everything works fine on production accounts. 

What we've already checked
- Credentials are copied exactly (no whitespace), and we point at the sandbox endpoint (apitest.authorize.net), not production.
- The problem appears on multiple, unrelated sandbox accounts simultaneously, which is why we don't think it's an issue with one specific account.

Is there any known sandbox-wide issue or recent change that would invalidate API credentials across multiple independent accounts at once?
Or maybe is there any account-status condition that can auto-invalidate keys for newly created/regenerated sandbox accounts?

[WilliamsWelsh]

One thing I'd also verify is whether the sandbox account itself has changed or been reset. Even if the API Login ID and Transaction Key haven't been modified in your code, regenerating the transaction key or creating a fresh set of sandbox credentials can sometimes resolve unexpected authentication failures. Also, make sure the requests are still being sent to the sandbox endpoint and not the production endpoint by mistake—this is a common cause of the E00007 error.