If you change the username that you use to logon to your CasaTunes server, CasaTunesSync will not find the correct folder to do its thing. To fix this, simply open CasaTunes Setup, click on the iTunes Plus tab and change the path in the “CasaTunesSync” folder box.
For example, if your username was “topgun” and you changed it to “casauser”, you will change the entry in “CasaTunesSync folder” from C:\Users\topgun\Music\CasaTunes to C:\Users\casauser\Music\CasaTunes