Since I moved to .NET 4.0 a few weeks ago, some of the tools had random crashes, usually at startup. It turned out this was an issue in the text editor control that is used in the tools. Mainly scenProc and FXEditor were affected, because the text editor is a main part of the GUI there, but ModelConverterX also uses it in some editors.
Yesterday I have fixed these issues, by disabling the buggy part of the control (it was a part we don’t use in the tool anyway). So please make sure to grab the latest development release.