This issue was killing us but we finally got it fixed with help from Chris of DNNCreative.

The issue was a 302 redirect to a 404 error page, which apparently was caused by an IIS setting. By resetting all the custom error pages to their default (even though they already appeared to be the default) the problem was resolved.

So, any time a child portal loses its login settings, begin by checking for any changes to custom error pages in IIS, or perhaps in the web.config of your installation. The strange thing was that not all portals we affected, so there may be more to the issue, but this fixed it for us.

Hope this helps others if you stumble accross this thread in search of help, as we could not find any other reference to this problem.

We also eventually had the same problem and fixed it the same way (the missing skin image).

Fortunately we don't have the store installed so that error didn't crop up.

As long as DNN can run without 302 or 404 errors, no one seems to get logged out.

In summary it seems any 302 or 404 error (missing resource) can cause DNN4.9 to lose authentication and log the user out (perhaps due to a redirect or something).