[...] Moreover, when I activated the debug mode, this MYSQL error was displayed: "Unknown column 'get_read_state_from_server' in 'field list'".
This is caused by the current upgrade procedure which requires two independent procedures to update the database. If you skip one, you are stuck with an inconsistent state of your installation.
Reason & Solution:
[Step1] The installation files (e.g. fengoffice_3.4.1.zip) contains the core files
including three preinstalled plugins "core_dimensions", "mail", "workspaces".
[Step2] The main upgrade script (fengoffice/public/upgrade) only updates the core database tables
without those of the preinstalled plugins.
If you stop here, your installation will most likely be broken, as the plugin files are updated but the database is not.
(in this case: 'get_read_state_from_server' is a new field introduced to the table 'mail_accounts' of the mail plugin. This table doesn't get updated by Step 2 so the mail plugin fails and breaks the installation)
[Step3] You can't use the GUI due to the error. Therefore you
must run the plugin update script (php fengoffice/public/install/plugin-console.php update_all) from console/command line to complete the database update for the plugins.
HTH
Thorsten