To make the Solution concrete in case anyone else comes across this issue:
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1200 PLCs have a different OPC structure from 1500 PLCs due to the setup necessary on the PLC side.
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On the 1500 PLC the checkbox “Standard server interface (SIMATIC namespace)” has to be enabled.
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“Legacy data type definitions” should be enabled on the OPC.
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Do not use the “Server Interface” method when communicating datatypes and structures on 1500 PLCs