Hi Kris1976,

for OPC you can create OPC trace to see the OPC error code.
The easiest way is as following:
1. Open the Program "Configuration console" in SIMAITC NET Start Menu folder. Enter Application -- OPC -- Trace and activate the Trace DATA. Over there you can set the trace level to error first.

2. If the OPC server returns an error code you will find it in this trace. It starts normally with 800***. Check the error code with the manual or easier you will have a look to the file OPC_ERROR.H in Simatic net installation path (make a search about this file, because I don't know the exact location of this file). Search about the error code and in the comments of this message you will find a description about the reason.

But on general, if you don’t change anything it would not change the result. So check what is happened. Additional connection to PLC added? Changes in the OPC application made? Changes in PLC Program made? Try to figure out what was happened.