I'm seeing what looks like the same fault, started abruptly at ~03:11 UTC on 2026-08-28 after weeks of working normally. Symptoms, matching yours almost exactly: Every private-database WRITE returns HTTP 500 with an empty response body. The client surfaces CKErrorDomain 15 (Server Rejected Request) with underlying CKInternalErrorDomain: 2001. Server header AppleHttpServer, via icloud-xrail. READS work. accountStatus reports available. Sign-in is fine. Reproduces identically through NSPersistentCloudKitContainer mirroring AND a raw CKModifyRecordsOperation probe, on device (iOS 17.2.1). Crucially, like you: I created a brand-new, fresh container and pointed the app at it — writes fail there too with the same 500/2001, so it's not container state. This appears to be account-scoped. Entitlements, provisioning profile, and App ID iCloud capability all verified correct (codesign inspection + ASC API). Development environment. One possibly relevant correlating event on my side: the legal name on my Apple Account was changed the same morning the failures began. I can't prove causation, but the timing is exact, and the failure being account-scoped (two containers, same signature) fits an account-state wedge in the write path — possibly the PCS/user-record layer, which would explain writes failing while reads keep working. Sample failing request UUIDs, if it helps anyone at Apple find these in server logs: 244DA811-E4AA-4771-88F0-7C092EFD8DEF (2026-08-28 ~09:57 UTC) 1FD544D6-6ED2-4181-8976-B4F4C35830D9 E55EFF8F-FC32-48BD-9165-F4EA263F130A I'm filing a Feedback and a support case referencing this thread so the reports get correlated rather than triaged separately. @OP — did case 21678909 / FB24394907 produce any movement? Happy to supply more request UUIDs or a CloudKit sysdiagnose (TN3163) if an Apple engineer picks this up.