I've done some more testing, Windows 7 x86 SP1 and 3.01-18 work. Seems like only 64-bit Windows 7 (most likely 2008R2 too) and 3.01-14->18 are not getting along.
I've added a summary to the download page for Windows clients: http://wiki.x2go.org/windows_and_macos_x_binaries. Some combinations aren't verified, please update if incorrect.
Might I also suggest that the outdated client download info on www.x2go.org (http://www.x2go.org/index.php?id=7) is updated with links to the wiki instead.
Cheers, Daniel