Chris Hammond, I reviewed the older thread you referenced and found it of interest, though one thing struck me and that is that this problem supposedly happens because of somebody checking "Remember Login", which I almost never do, and can confirm that I have not done with any instance of DNN on my local host. I have also never run two instances of DNN on the same installation; I've always run each instance independently. Also, if things are as described with respect to cookie clearing, then surely my having cleared the cookies would stop the problem, but it hasn't.
But I tried an experiment that suggested itself from what was said in that other thread and what I know of the Rainbow portal.
I'm running the Rainbow portal (trying to transfer a lot of content from it to DNN), and if I am not incorrect, Rainbow, although a C# product, was apparently ported from an earlier version of DNN (or its predecessor, IBuySpy) and thus may share a similar cookie scheme. I DO "remember login" with the RB portal. Since I have not been running DNN without running RB at the same time (while attempting to transfer content), I thought there was an outside chance of interference. Clutching at straws, I thought, since RB has never barked at me with the cryptographic exception thing.
I made the experiment as follows: I made sure to log out of Rainbow, then cleared cookies. I brought up DNN and began throwing a lot of editing at it: adding pages and modules, editing content, just going to town on it. No cryptographic exceptions whatsoever, and I had never before been able to do so much work without the exception coming up and in pretty short order. After a while I went ahead and brought up Rainbow and signed in, remembering the login. I then went back to DNN and tried to edit something I had successfully edited before and WHAM. First thing right out of the box I got my cryptographic exception. I then logged out of Rainbow and tried to edit a text/html box, and no problem. But as soon as I logged into Rainbow I would get the exception in DNN, which would go away once I logged out of Rainbow. Highly repeatable. At least with the version of RB that I have been running.
I've done this now enough times that I am dead certain that RB and DNN do not mix on the same machine. You don't even need to be remembering the login; it happens either way.
So, have I discovered something brand new? If so, I feel especially privileged, even though it is going to make transferring my content to DNN a bit harder.