Thanks both, very helpful. Two updates.

First, a correction on my side: the workspace is in us-east-2, not London, so cross-Geo processing should not have been required for this region.

Second, I checked the workspace Security and compliance tab anyway. Compliance security profile is disabled. "Enforce data processing within workspace Geography for Designated Services" was enabled, so I turned it off. Enable content search still fails right after with the same error (it may need time to propagate, I will retry later).

Details in case they help: the UI call is PATCH /ajax-api/2.1/unity-catalog/volumes/{volume} returning 400. GET on the volume with include_content_search=true returns no content_search object at all. The volume is a managed volume on Default Storage (securable_kind VOLUME_DB_STORAGE), catalog binding is open.

If it still fails after the setting propagates I will raise a support case with the request IDs. Will report back.