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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.

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)
    }
}