Hi,
This error indicates that the program was denied access to the necessary file (in this case, the file of market settings) by the operation system. There are numerous reasons why it could happen, and all of them are outside the scope of MarketFeeder Pro. One of them, for instance, is having the folder with the events opened in a file browser in a way that blocks the rewrite of the files. Another reason could be an antivirus program performing some activity on those files.
I wouldn't worry too much, it did happen with me as well in the past, but it never became a recurring problem.