EnviroInsite distributes several EarthSoft files with the application. These enable us to take advantage of all of the controlled access capabilities within EarthSoft. Based on discussions with you, it's clear that your installation of EQuIS 6.4.1 is ahead of the files that we are distributing with EnviroInsite. In the past this hasn't been a problem, but apparently with this EQuIS release there's something that breaks the compatability with our files.
As you discovered copying the EarthSoft.Common.dll, EarthSoft.EQuIS.dll and EarthSoft.Registration.dll files from the EQuIS application folder into the EnviroInsite application folder resolved the problem. Very resourceful of you and thanks for the info.