I'm not sure what this problem will be then. I have occassionally come across this in earlier versions of DNN and it seemed to be fixed in DNN3.2.2 - I haven't come across the error in quite a while. - The fix was to just delete cookies and then all was fine.
This could be a problem with DNN4.3.1 as it is still in the alpha stage. - You could try upgrading to 4.3.2 - actually I would recommend this anyway as they have fixed a lot of the bugs.
(If you have not upgraded before, check this tutorial:
How to upgrade DotNetNuke from v3.x to v3.2 on a live websiteit will give you a lot of pointers that you can use for v4)
Do you have firefox on your computer? - I would also try firefox as I found that IE was more prone to the login problem than Firefox.
The thing is though, even if you can login with firefox if all of your other users start receiving login problems, you can't tell them all just to use firefox!
I would try firefox though as a test, thanks,