Redmine

Hi everyone,

I managed to get Redmine installed after some efforts and some troubleshooting, but there's one problem I could not solve regarding the scrum plugin. Here's some info about the installation. Thanks in advance for any hints or solution.
Denis

SYMPTOMS : In the web interface > administration > plugin > scrum : configure, whatever I select in the pull down menus is not preserved after I click Apply. The form just resets empty.

CHECKS : In Admin > information, The public plugin folder is writeable.
In log/production.log or in apache2/error.log, there are not errors.
I enabled debug in config/additional_environment.rb but there are no obvious errors in the resulting log (at least to my eyes).
(the only thing may be suspicious is a ? in the sql queries ( WHERE `custom_fields`.`id` = ? ).

The whole /usr/share/redmine folder and subfolders are owned by www-data.
public and plugin are chmod 755

CONFIG: /etc/apache2/sites-available/redmine.conf

<VirtualHost *:3000>
ServerName redmine.<our_internal>.<domain>
RailsEnv production
SetEnv X_DEBIAN_SITEID "default"
PassengerDefaultUser www-data
Alias "/plugin_assets/" /usr/share/redmine/public/plugin_assets/
DocumentRoot /usr/share/redmine/public
<Directory /usr/share/redmine/public>
Options ExecCGI FollowSymLinks
Options -MultiViews
Require all granted
</Directory>
<Directory /usr/share/redmine/plugins/scrum/>
Require all granted
</Directory>
</VirtualHost>

ENVIRONMENT: Here's a copy of administration > information

Environment:
Redmine version 3.3.0.devel
Ruby version 2.1.5-p273 (2014-11-13) [x86_64-linux-gnu]
Rails version 4.2.7.1
Environment production
Database adapter MySQL
SCM:
Git 2.10.1
Filesystem
Redmine plugins:
scrum 0.14.0

!! new:
updated to scrum0.15
problem persists