Very good news... I've solved the case.
The issue is with another plugin called: "User - RaidPlanner"
It's a plugin used with the component "Raidplanner" for Joomla.
If I enable the plugin, I've the issues reported before.
If I disabled the plugin, all works !.
As this is not a CB related issue, I'll take this further on with the developper.
A big thanks to nant, who helped out identifying the issue.