First of all I forgot that the systems is OSX 10.67 Server. Sorry about it...
System Preference > Sharing > Remote Management setting are for specific users, but if set for all users the
behavior is the same.
I run "verify permission" from Disk Utility and I've found a relevant message
" Warning: SUID /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDag ent/" has been modified and will note be repaired.
I was not able to find any message on logs about this problem.
In server admin, click the server in question.
Click the "Access" button at the top of the window.
Make sure you're looking at the "Services" tab.
Click the "For all services" button.
Click the "Allow all users and groups" button.
If it's already set that way, then there's a problem with your remote desktop client application on the computer.