Migrating clusters from ECE 1.1 to ECE 2.6

My team is attempting to come up with procedures to migrate clusters created on ECE 1.1 to a new installation that will be running ECE 2.6. We're trying to use remote snapshots. Our ECE 1.1 clusters have custom base paths, so we're copying that into transient.restore_snapshot.repository_config settings in the create deployment payload on ECE 2.6. The JSON validates and the create works until we receive the following error:

Screen Shot 2021-02-22 at 9.37.08 AM

Has anyone seen this error? We specified the type in transient.restore_snapshot.repository_config, but we still get the same error. We also tried specifying only one index, but that also failed with the same error.