The first question to ask is it a genuine UC100 from CNCdrive motion controls or was it bought from eBay.?

Next, I've seen this happen many times with various controllers, and often it's a corrupted Mach3 XML. Either create a new profile using the same settings or go into the XML backup folder and use an old one that you know worked. Personally I'd set up a new one.

Or better still invest in UCCNC as it's now much better than Mach3.