Cowork "Add marketplace" returns generic error while CLI succeeds on same account

Environment:

Cowork: 1.8089.1 Claude Code CLI: v2.1.150 OS: Windows 11 (clean reinstall on 2026-05-23) Account: single-user personal org, no managed policies orgId: available on request (will share via private channel with engineer assigned)

Summary: Adding the anthropics/claude-for-legal marketplace via Cowork's Add Marketplace dialog fails with the generic message "Failed to add marketplace" (no error code, no detail). The same marketplace adds successfully via Claude Code CLI on the same account (/plugin marketplace add anthropics/claude-for-legal returns "Successfully added marketplace: claude-for-legal"). After CLI registration and full Cowork restart, the marketplace does not appear in Cowork's Personal plugins, and re-attempting via the Cowork dialog still returns the same generic error. Reproduction:

Open Cowork → Personal plugins → Add marketplace Enter anthropics/claude-for-legal (also tried full HTTPS URL, with/without capitalisation, with .json suffix) Dialog returns "Failed to add marketplace"

Failure timestamps (UTC), for server-log lookup of remoteMarketplaceClient.createAccountMarketplace:

2026-05-21: 22:12:33, 22:34:12, 22:43:24, 22:43:37, 22:44:09, 22:44:22, 22:44:52, 22:46:16, 22:47:03 2026-05-23: 16:19:00

Already eliminated: local cache, full uninstall + reinstall, Windows Hypervisor Platform, network/VPN, URL formatting. Support triage confirmed organization policies do not apply to a single-user org and the marketplace is Anthropic's own public release. Workaround: Plugins are usable via CLI.