After closing the designer I looked into the console and I see that is it flooded with messages like the one below.
Should I be worried?

[quote]10:31:08 AM DeleteMonitoredItemsServiceCall Server returned StatusCode[Severity=Bad, Subcode=Bad_MonitoredItemIdInvalid] for item ‘UaSubscriptionItem{nodeId=ns=2:s=Twincat1.CX_Waterz.OPC_IB2722_MBU_FIT03_LuchtDebiet_MBU_All.TVL[1], itemId=4662, clientHandle=24751}’.

I don’t think you need to worry about this - it should be getting fixed soon, if it’s not already, but basically what’s happening is that in a certain case the subscription model is tricking the UA client code into deleting items it already deleted. Looks scary-ish, but it’s a bookkeeping error that doesn’t negatively effect you overall.