I solved it!
The error was that I've been working with the zeo before and now with standalone installation, so I had to change buildout users from plone_daemon to plone_buildout on the buildout.cfg file:
[buildout]
############################################
# Plone Component Versions
# ------------------------
# This version of the Unified Installer has the components of Plone 5
# preloaded so that it can install without an Internet connection.
# If you want to update, uncomment the "http://..." line below,
# edit it to point to the current version URL, comment out the
# "versions.cfg" line and run "bin/buildout" while attached to the
# Internet. Generally, you only want to do that as part of a planned migration.
# Note that if you are updating components, you should also check the versions
# section at the end of this file, since recipes or components other than
# those of Zope and Plone may need updating at the same time.
#
extends =
base.cfg
versions.cfg
# http://dist.plone.org/release/5.1.0/versions.cfg
# If you change your Plone version, you'll also need to update
# the repository link below.
find-links +=
http://dist.plone.org/release/5.1.0
# If you try to start Zope as root, it will change user id to run as
# the effective user specified here. This user id must own the var directory
# of your buildout.
**effective-user = plone_buildout**
# This user will own the rest of the installation, and should be used to
# run buildout.
**buildout-user = plone_buildout**
# A flag to tell the Unified Installer whether or not to document sudo use.
need-sudo = yes
After I found out that some directories had wrong permissions:
[/opt/plone510a/zinstance/src]# ll
total 16
drwxr-sr-- 5 root plone_group 4096 oct 8 14:00 plone.mysite
drwxr-sr-- 5 root plone_group 4096 oct 8 15:20 plone.importexport
-rwxr-xr-- 1 plone_buildout plone_group 62 oct 8 13:55 README.txt
drwxr-sr-- 3 plone_buildout plone_group 4096 oct 8 13:55 z3c.recipe.staticlxml
And I fixed it doing:
chown plone_buildout:plone_group -R plone.mysite/
chown plone_buildout:plone_group -R plone.importexport/