Hi!
I have also come across exactly the same problem with my ISP (it works on my localhost).
They say that it wants to run with "Full Trust" (and they only allow "Medium Trust").
They also say that they don't allow Assemblies that haven't specified that "Partial Trust" is allowed (which I understand means that the addition of [assembly: AllowPartiallyTrustedCallers] was a good thing, but it seems to not have helped?)
I have also tried all previous versions of IZ.WebFileManager.dll, but still the same error message.
It's a real pity, since I think this file manager is the best there is. And there must be a lot more than just a few of us that are experiencing the same problem?