You should always communicate with the server at least every 60 days otherwise the server assumes that you have dropped those assignments and gives them to someone else. Since you are only a couple of days from completion, I would suggest that you add the following lines to your worktodo.txt  after stopping Prime95 
Test=N/A,xxxxxxxx,68,1
where xxxxxxxx is the exponent in question. This will allow you to complete the exponents and return the results without getting the invalid key error. Note that in doing this, you are completing an exponent that is now assigned to someone else  you can check the status of your exponents from the exponent status page.
