RS232 has relatively high signal levels (+-9V) that would fry the DS28E17. You want a 1-Wire capable MCU or a USB adapter. It can be done with an Arduino.
You want to create a scan list. At 10Hz, that's 160 relay close, stabilize, measure, relay open operations. You might have to sacrifice resolution to get to that speed.
Is PCIe a requirement? FTDI (assuming you get them from reliable sources) produce very reliable USB to RS232 cables. They also have TTL and LVTTL versions.
LAN - Did you set up the R&S IP address? Do you get Ping replies?
USB - Did you install the USB drivers from R&S? does the device appear in Windows device manager?
What is the model of the SA?
When writing arrays directly to Excel, there is a time limit for Excel to write the array to the selected range. If that limit is exceeded, you will get an error.
The .NET API provides a more detailed error than the ActiveX API.