I have the same problem. I'm dloading the tar.gz on a mac. My hosting will only extract .zip files, so i repackage the tar.gz as a .zip and then upload to my hosting service. I do not have access to ssh. Clearly, the native os x decompressor is having the same issue as IzArc, since the error msg is exactly the same.
I don't really know how else to do this. I could get the latest from source control, but I'd feel more comfortable using the latest stable.
Just a little more info here. I have unpacked the tar.gz in Linux and then repackaged as a .zip. The problem has not changed. =( As far as I can tell, IzArc is not the problem, and unpacking in Linux does not fix it. Perhaps there is a problem with the tar.gz that is currently being distributed?