Resolved — a fix has been deployed. Generating new addresses from extended public keys (HD wallets) is fully operational again. No data was lost and no action is required.

We're investigating an issue where generating new addresses from extended public keys (HD wallets) on the Platform API may fail. Transaction monitoring, balances, RPC, and all other functionality are unaffected, and no data has been lost. A fix is in progress — we'll post an update shortly.