So, I did some poking around in the Control By Web Support page for the X600-M (the manual I sent in the OP). I then talked to support for CBW and found that the register addresses are always 2-bit for holding registers. So the solution came out to what Matt J. said, which is that Write Single Register is not supported, because each address is "two addresses". For example, my first register is called address 0002, but is actually 0001 and 0002. So I replaced the Write Single Register with Write Multiple Registers, and ensured it wrote a value to two addresses.
Thank you both for your help.