Hi sounds like a permission is at fault here, go into the home folder and CHMOD the folders ensuring that they are readable, and executable. Additionally you could add the user to the www-data and see if that resolves the issue but I doubt that's the case.
Ensure that you are using the -R command on the chmod this will set it to recursively do all the folders and all the files. Since 755 on folders doesn't unlock the actual files itself and they will remain unreadable.
sudo chmod 755 -R /home/username/lamp/htdocs