We worked on this in the support case you raised and you mentioned that your instance of this issue/error is now resolved. If you have any further questions regarding your issue, please respond in the support case for continuity.
For anyone else that experiences this issue, the bad changeset (DOWNLOAD) error is usually a result of (but not limited to):
Schema inconsistencies:
Compare your models between the client and the Realm schema. Are there any differences?
Did you try to change your client models with development mode disabled?
Did you make destructive changes? Even with development mode turned on, these type of change will not automatically be mapped to the schema for synced objects.
Are there any errors in your Realm app? Check your Realm logs via the UI and filter by status: error to see if there are any Schema errors that will help target the root cause further.
Have you tried terminating sync and re-enabling it? Have you tried deleting the app from device and running it again? This is sometimes needed after changes are made to the Schema.
Are there any partition related sync errors in your Realm logs? You may see something like incorrect partition value for key ____ expectedPartition: ____ foundPartition: ____