The problem could be that both OC and Dfix have the option to browse board without cookies option off. I neer have these problems and this is the one difference in our account setups. You can modify this is the edit options part of USER CP if you want it on.
With this ON you should not have as much trouble as it is easier for the board to track you if you have the cookies turned on.
You may not want to have the cookies on but in the end it will give you a better user experience and log off problems should not happen as often.