@TonyW-
Yeah, try the other version of the GUI, if you can. (ReAction and ClassAct are the same thing)
I had used Miami under OS 3.1/3.5/3.9 for a long time with great success (other than being forced to disable DHCP!). I faintly remember the problem you were describing, too. Can you manually edit a config file outside the GUI? I either did that, or switched GUIs.
The problem I ran into later was that my X-Surf died, and Miami didn't properly support the C= network card I got to replace it. Genesis works great, though.