I just tried and I can enter any value I want. It's a string so it should accept anything you enter. However, I will say that I don't see a way to bind to a driver, so that's a problem, assuming you have more than one on a form.
Under Miscellaneous (Misc) there is an option called
ComComponent, there is where you assign the driver. Just out of curiosity I changed the option to none and sure enough it allowed me to enter address 40001 with no errors. I then tried changing the driver back to Modbus TCP and what do you know? It is now working!
I deleted the Circular bar component, introduced a new one, entered address 40001 and the error no longer appears.
So don't know what the issue was, but I seem to remember that when I was getting the error there where no options under the PLC Address Value field and after what i did, mentioned above, I now see several options.
Whatever it was it seems to be working now.