Thanks for your thoughts makeportal, frankt and proeder.

Have tried adding impersonation = "true" to web.config - no difference unfortunately. Host won't change from medium trust level, and I'm asking them about asp.net priviledges to the root folder of the hard-drive. But I don't have much hope.

Strange thing is that the error is not consistent. There are times when, after getting the error, I can go back a couple of pages in the browser and refresh OK, and then go successfully to the page that failed previously, but the when I go to another, the error reappears!

Thanks frankt. Host is http://www.webcentral.com.au

DNN is installed in a sub-directory called /dnntest, which is set up as an application root. I also had a separate install in /dnnprod with a separate SQL database, but have deleted that now as the host thought two installations may be part of the problem.

Webcentral seems to have solved the security exception error - I spoke with one of their systems engineers who said they used a thing called Reflection (or something like that) to fix that problem. I'll ask them for more info about this for the others who have this problem.

However, the site is now running like a dog (very slow) and I'm getting the odd "Object reference not set to an instance of an object" in the log, but these don't seem to interfere with its use.