Screen sharing not working in mountain lion

What you mean by screen sharing ?

Screen Sharing works just fine for me both ways with 10.7. What happens when you use the "Connect to Server" window? With finder in front, click the "Go" menu and select "Connect to Server". In that window, add text to the field looking like this:

vnc://TargetMachineName.local

Do you get any kind of login window?

Have you updated the Remote Desktop software to 3.6 on both computers?

Having similar issues. AFP sharing works but screen sharing is failing between cetrtain machines. Works after I restart target Mac, but then will fail later. Do not get error message. VNC: fails same way. No message

Works fine for me using 10.8 to 10.8, 10.8 to 10.7 etc - have you enabled BackToMyMac in iCloud preferences, and then in sharing settings checked screenshare - you will need to enable this for both Macs.

Still having issues. I must have some kind of port blocking enabled, but cannot figure out.

Mac A is 192.168.0.101

Mac B is 192.168.0.100

Mac A can screen share or ARD with Mac B and other machines on network

Mac B cannot screen share or ARD with Mac A, and other machines on network also cannot screen share with Mac A

All machines can file share with Mac A however.

Also worth noting that reboot of Mac A seems to correct the problem for a time, but eventually connections get blocked again

I was having the same problem and what worked for me was starting with one mac: I selected the screen sharing and file sharing (with default setting... no changes) in the System Preferences -> Sharing. I then deselected and reselected the Back to my Mac in Preference -> iCould, then restarted. I followed this process on both my MBP and Mini and the Screen Sharing works great now

Same issue, formerly working fine now working on LAN but not WAN.

Interestingly, Back to My Mac in iCloud panel of system preferences keeps reporting that UPnP is turned off on my router though I have confirmed that it is in fact on (and rebooted router several times).

I repaired permissions on target mac and removed a corrupted file from preferences (com.apple.VNCpref.txt). Now working.

I'm having the same issue randomly as well on a local LAN behind an Airport Extreme base station.

I have a mixed environment of Mountain Lion, Lion and Snow Leopard. I'll be able to screen share fine most of the day but then there will be hours when non of the machines will screen share with each other.

Restarting doesn't help.

This didn't happen before 2 of my systems were upgraded to Mountain Lion.

I did some testing.

When I set up a separate LAN and connected a Leopard and Snow Leopard system, they were able to screen share just fine. When I put them both back on the regular LAN, they were unable to connect.

Then I disconnected the 2 Mountain Lion systems from the LAN and once again the Leopard and Snow Leopard systems could screen share with each other again.

I don't know what Mountain Lion is doing, but the results are like a poision the LAN.

And tonight I am suddenly able to screen share to all the Macs on the LAN.

I have made no changes.

This is a very strange issue.

Have you tried to repair permissions on target mac or macs? That is what worked for me

Hm; I am also having trouble connecting to VNC using Bonjour addressing (vnc://user@machinename.local). Clicking on the "Share Screen" button when the machine is selected in the Finder sidebar also fails. However, using the actual IP address of the machine is working (vnc://user@192.168.1.x).

The odd thing is that while I had installed ML on this machine, this is occurring while actually running Lion, and no ML is running on my network (the machine I'm trying to connect to is on Snow Leopard). I had not seen this issue before installing ML last week. If this does have anything to do with ML, it doesn't require ML to be actually running.