Had some problems installing themes in WordPress. After a default installation, I went to install a theme using the built-in download manager. The system asks for a username and password. This login must be created in the system and allowed to FTP.
Because I am using the Plesk control panel, I went to my domain settings and added a FTP user.
Even after I created this user, I saw an error indicating that I could not create the wp-contentupgrade directory. After a bit of research, I noticed all the directories were owned by ‘root’… So, I simply executed a command as follows
chown -R FTPUSERNAME wp-content**
Also, make sure that your PHP Safe Mode is OFF.