Hmm, I think you might need to contact Beckhoff for support, or start with a firmware upgrade if something like that is available.
We create the Monitored Item and it get assigned an id of 303:
Then in the subsequent attempt call to SetMonitoringMode the server thinks it doesn't exist:
Some clients create the items in MonitoringMode.Reporting to being with, some create them in Disabled or Sampling mode and then flip them to Reporting when ready. Ours does the latter, which is why the SetMonitoringMode calls appear.