Right, Kai has found the solution, and told me how to sort out the problem.
Basically, if your using MySQL to authenticate ProFTPD users (as described and explained by Stephen Khoo - http://www.khoosys.net/single.htm?ipg=848
), you also have to change file/folder owners and groups by using the chown command.
For instance, if you want to give access to /opt/lampp/htdocs, you have to type:
chown ftpuser /opt/lampp/htdocs
ftpuser being the user defined in MySQL (see Stephen Khoo's thread at http://www.khoosys.net/single.htm?ipg=848