(I've been meaning to post this info somewhere.. so this thread will have to do)
I spent untold long hours and made many posts trying to figure this issue (the thread) out, but to no avail. At one point I had it pinned down to when I installed and activated a replacement icon bar called DNN Control, but some time conversing with the developer and many more installs nixed that.
In the end it is working for me now... many portals, no errors. I have one thing that I had suspected early on as a possible cause of the problem but have not fully confirmed it. All I know is that ever since I stopped using the MSN toolbar for filling in passes and forms in IE, the problem hasn't happened.
I actually wrote about this to MS back when the Toolbar first came out. At that time I had just been through a change of spam filtering on a few dozen email accounts only to find afterwards that all the passwords had been changed to my own. Imagine the trouble trying to get 2 dozen non-IT-savvy clients spread around the world to change their passes back to what they were... and the channel for communicating with them, email, had just been cut off!! Mayhem!
I took this to be a serious flaw in the operation of the toolbar and indicated this to MS, but they never responded.. so I thought; stuff them... let them find out about it in a more serious manner elsewhere.
In any case.. I think the toolbar either fills in hidden inputs or does something similar that affects the content of the form.. and all our pages in DNN are of course forms. This can also happen when you have collapsed form fields as are common in DNN. Another suspicious tip-off was when Firefox where I only have the developer toolbar installed, once asked me which of two passwords on the page I wanted to update... yet there was only one password that I could see anywhere on the page.
Sorry for the long reply.. and it may not fix your problem.. turning the toolbar off doesn't just fix the error; I had to install from scratch and then never use it again.. hence the reason why I haven't confirmed the problem since - I'm not going to turn the toolbar on and find out.
Try it and let me know what happens.
Rob