I haven't seen this error before. A bit of searching on Google shows that others have had that error with GTKRadiant, but it usually happens with Vista. (UFORadiant is a fork of GTKRadiant, so it has much of the same base.)
The one thing people talked about was right-clicking on the .exe, selecting properties and then trying different compatibility settings.
If that doesn't help, try making a fuller bug report over at UFOAI's sourceforge page. If you can, include details on what version you're running (is it the uforadiant that shipped with the official 2.3 installer?), and include the text of the log that appears in the window at the bottom of UFORadiant. If you can't see that log window, hover your mouse over the very bottom of the lower grids and you should be able to raise those windows up, revealing a little log window.