screen sharing error

I want my MacBook Pro to share its screen with my iMac. Both OSX 10.6.8; both on Wifi (n). It always went fine. Since a few weeks it won't work and I get an error on the iMac when I call there for the MacBook's screen.

I tried switching off Screen sharing on the MacBook, restarting it, and then turning it on again. To no avail.

I have 3 Macs, all on 10.6.8. The third is a Mac Mini; the Mini can see the screen of the MacBook very well. So, the problem is just the iMac not being able to see the MacBook's screen.

Where should I look to solve this?