Wordpress Update fails - but why?

[onycro]

This question has its origin at stackexchange - superuser, but they've told me to rather ask this here in the support forum. So here it goes...


I am currently using wordpress 4.4 I set up background updates but when I execute them it says 'Something went wrong. Unable to locate WordPress Root directory.'

I set my FTP details like that:

define('FTP_HOST', 'mydomain.com');
define('FTP_USER', 'user');
define('FTP_PASS', 'passwort');

When I connect to this server using this login details in a normal ftp client, it works fine. I am able to locate the wordpress directory then.

I also was unable to enable debug logs. I set

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', true );

But in wp-content the logfile is missing debug.log. I thought it would depend on the file permissions:

drwxrwxr--  6 myuser www-data   4096 Feb 27 16:13 wp-content

But those are set correctly, as I can see that the webserver which runs wordpress is in the group www-data.

> ps auxww|grep http
www-data  3657  0.0  1.2 458240 52088 ?        Sl   06:03   0:03 /usr/sbin/apache2 -k start
root      4999  0.0  0.0  15800   912 pts/2    S+   10:53   0:00 grep --color=auto apache
root      7396  0.0  1.0 451200 40304 ?        Ss   Jan27   1:25 /usr/sbin/apache2 -k start
www-data 21645  0.0  1.4 465896 56320 ?        Sl   Feb26   0:36 /usr/sbin/apache2 -k start   
> id -a www-data
uid=33(www-data) gid=33(www-data) groups=33(www-data),1002(psaserv),1005(horde_sysgroup),1007(psasb)

So I have two questions:

1. Why is wordpress unable to locate the root directory?
2. Why does wordpress not create the logfile?

Additional info:
- my plesk version: 12.0.18
- I did not use the automatic plesk installer to install wordpress and it is a new (manually-)installed version