You may want to try deleting the dat file like this. Close DW, go to the dat file and delete it, then open DW again. Before making use of anything close it again and go back to the dat file and delete it one more time. I had something like this happen to me the other day and I was able to resolve it by deleting the file twice.
If this does not work for you then you may have a corruption of your configuration folder. Here is a link with the instructions for this:
Please post back and let us know if you continue to have problems with this so we can look into the issue further with you.