As a follow-up, it would appear that you can't use any of the date fields to run the ISync. Eliminating that from the parameters seems to have "resolved" the issue.

Thanks for sharing your solution.

Here is a simple example how to sync the message store in Node JS.

syncExtensionMessageStore("")



async function syncExtensionMessageStore(syncToken){

    console.log("===== sync_extension_message_store ======")

    var endpoint = '/restapi/v1.0/account/~/extension/~/message-sync'

    var params = {}

    if (syncToken == ""){

      params = {

        dateFrom: "2021-08-02T23:59:59.999Z",

        syncType: 'FSync',

        recordCount: 250

      }

    }else{

      params['syncToken'] = syncToken

      params.syncType = 'ISync'

      params.recordCount = 250

    }

    try{

      const resp = await platform.get(endpoint, params)

      var jsonObj = await resp.json()

      //console.log(jsonObj)

      if (jsonObj.records.length > 0){

        setTimeout(function(){

            syncExtensionMessageStore(jsonObj.syncInfo.syncToken)

        }, 2000)

      }else{

        console.log('Done!')

      }

    }catch (e){

      console.log(e.message)

    }

}