Guys after messing around with the ACL in all other places, and checking for any kind of possible other issues for some hours, finally I managed to sort this out.
The issue was caused by the ACL settingsACL settings on the JCE pluginJCE Editor plugin, which is being called when it comes to load the editor. It had access level to Administrator, so any front-end users didn't have accesswere unable to itget access and load the editor.
I set this toits access to all front-end usersPublic and now I have the editor workingloading on Front-end for all my usergroups.