Niborius said:
Nope, i'm not 
When you login some unique bits of info are generated to identify you. One of these is based on your ip address, the rest are generated randomly by the server.
These bits of info are stored in a cookie in your browser, and in the database.
Each time you browse to a new page these bits of info are retrieved from your cookies and validated against the values stored in the database for your user account.
If the information does not match you get the "oops, there was a problem..." message.
I can't understand where in this process things are going wrong for you. If you login in one browser, login in a second browser and then go back to the first browser you'll get the message because the login in the second browser will have changed the info in the database... but if you aren't changing browser or changing IP I can't understand where the mismatch could be occurring. Possibly there could be a problem setting cookies on your browser, but I don't really see how this could be.
I can't replicate in FF3.0.10 (which is what you're using, you gave the .exe version), nor any other browser. Noone else has mentioned any issues, and the login system hasn't really changed significantly since last summer :/
Very sorry but I'm struggling to understand what is going on and so can't do anything to fix things unless you or someone else could suggest any other possible factors?