Log files, which used to show up in the MyDefrag folder, are now showing up in my %USERPROFILE% folder.
MyDefrag writes the logfile and .dat file to the MyDefrag installation folder. The default folder is "C:\Program Files\MyDefrag v4.3.1", but you can change this when installing MyDefrag. Did you do that? If so then uninstall MyDefrag and install again. Make sure you do it on a userid with administrator permissions.