Plan per Resource exception class '' not found

Hello all
Recently installed v6.2.4 to manage a customer's projects portfolio:
- around 100 projects with 2 to 5 activities per projects
- 20 resources (team members) may be assigned on these activities

I started assigning a few resource to few activities and compute project and resource planning... it worked fine a few times until now. When I'm selecting "Suivi > Planning par ressource", then selecting "all projects" and a specific resource, clicking on the button to calculate the resource's work plan, I get an exception: "Class ' ' not found" .

Log is not really helpful:
2017-05-30 23:12:44.585 ***** ERROR ***** [V6.2.4] EXCEPTION *****
2017-05-30 23:12:44.585 ***** ERROR ***** [V6.2.4] on file 'C:\xampp7\htdocs\projeqtor\model\persistence\SqlElement.php' at line (4833)
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] cause = Class '' not found
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #0 C:\xampp7\htdocs\projeqtor\model\persistence\SqlElement.php (3403) -> getExtraRequiredFields()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #1 C:\xampp7\htdocs\projeqtor\model\PlanningElement.php (850) -> control()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #2 C:\xampp7\htdocs\projeqtor\model\persistence\SqlElement.php (646) -> control()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #3 C:\xampp7\htdocs\projeqtor\model\persistence\SqlElement.php (542) -> saveSqlElement()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #4 C:\xampp7\htdocs\projeqtor\model\PlanningElement.php (560) -> save()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #5 C:\xampp7\htdocs\projeqtor\model\PlannedWork.php (822) -> simpleSave()
2017-05-30 23:12:44.601 ***** ERROR ***** [V6.2.4] => #6 C:\xampp7\htdocs\projeqtor\tool\plan.php (46) -> plan()

Did you ever got this issue and any idea to solve it please?
Thx