I/O access error, reading

In Simatic Manager, select the CPU, then click "Hardware" in the window at the right hand side of the screen.
The hardware configuration opens. You can see the station addresses for all the slaves on the profibus network. Select a slave, and in the bottom of the window you can then see the I/O addresses assigned to the slave.

I recommend that you use the Hardware Diagnostics to get more detailed information about the fault.