Unexpected Error

I've a found an issue like other posted here
I'm on Ubuntu 14.04 & MariaDB 10.x
Admin user login without issue
All other users has this error:
Type: Errors
Severity: Error Error
Date: 29/05/2016 16:02
Message: Unhandled System_Db_Exception: in /var/www/html/wi.ashnet.it/html/system/db/mysqli/engine.inc.php on line 287
Stack trace:
#0 /var/www/html/wi.ashnet.it/html/system/db/mysqli/engine.inc.php(110): System_Db_Mysqli_Engine->handleError(Object(mysqli))
#1 /var/www/html/wi.ashnet.it/html/system/db/mysqli/engine.inc.php(90): System_Db_Mysqli_Engine->executeStatement('SELECT COUNT(*)...', Array)
#2 /var/www/html/wi.ashnet.it/html/system/db/connection.inc.php(256): System_Db_Mysqli_Engine->query('SELECT COUNT(*)...', Array)
#3 /var/www/html/wi.ashnet.it/html/system/db/connection.inc.php(356): System_Db_Connection->queryArgs('SELECT COUNT(*)...', Array)
#4 /var/www/html/wi.ashnet.it/html/system/db/connection.inc.php(343): System_Db_Connection->queryScalarArgs('SELECT COUNT(*)...', Array)
#5 /var/www/html/wi.ashnet.it/html/system/api/projectmanager.inc.php(677): System_Db_Connection->queryScalar('SELECT COUNT(*)...', 11, 2)
#6 /var/www/html/wi.ashnet.it/html/client/projectstree.inc.php(66): System_Api_ProjectManager->getProjectsCount()
#7 /var/www/html/wi.ashnet.it/html/system/web/component.inc.php(100): Client_ProjectsTree->execute()
#8 /var/www/html/wi.ashnet.it/html/system/web/view.inc.php(247): System_Web_Component->run()
#9 /var/www/html/wi.ashnet.it/html/client/index.html.php(22): System_Web_View->insertComponent('Client_Projects...')
#10 /var/www/html/wi.ashnet.it/html/system/web/view.inc.php(284): include('/var/www/html/w...')
#11 /var/www/html/wi.ashnet.it/html/system/web/view.inc.php(217): System_Web_View->executeTemplate()
#12 /var/www/html/wi.ashnet.it/html/system/web/component.inc.php(102): System_Web_View->render()
#13 /var/www/html/wi.ashnet.it/html/system/web/application.inc.php(77): System_Web_Component->run()
#14 /var/www/html/wi.ashnet.it/html/system/core/application.inc.php(210): System_Web_Application->execute()
#15 /var/www/html/wi.ashnet.it/html/system/bootstrap.inc.php(130): System_Core_Application->run()
#16 /var/www/html/wi.ashnet.it/html/client/index.php(61): System_Bootstrap::run('Common_Applicat...', 'Client_Index')
#17 {main}

after user can work without problem, i've seen this only a session log problem..

Some help to fix ?