Igor,
I decided now not to purchase an incident to let your support check the issue on my server. The reason is easy: I found that there are many issues with selinux, e.g. if using Wordpress FTP updates always times occur, that the update fails because of anything new with proftpd provided by Plesk (which per default does not work with selinux, first need to run many audit2allow adjustments), so I decided to perform the updates setting wp-config to do changes directly to the files on the server instead of using FTP. Also creating hostings with plesk fail e.g. on changing the rights for custom error docs because of selinux.
So I know, there are many quality issues with Plesk (again, but currently more buggy than even before, using it in hosting since version 8), so that I found a plugin, which is doing the job of automatic updates also of plugins and themes for me, that's enough for me, I don't really require the wordpress option in Plesk. I already just upgradet to the Web Pro from the Web Admin edition just because of missing possibilities to access the Health monitoring, promised to be fixed from you months ago.
As this issue with Wordpress seems to occur for many others (there are additional threads in the forum about that) and proven with a fresh install, like TomBoB states, it happens, that the link to Plesk fails just first time adjusting anything in Wordpress, also seems not to be an issue of CentOS 7 or selinux, I would be happy to provide you access to my system to find the bug and fix it in future versions, but I'm not willing to pay therefore in front.
I will use the Easy Updates Manager plugin for anyone, who like to know.
Regards,
Christian