I have 4 Mac's in my household, two MBP's, a Mac Pro, and a Mac Mini. I have to up a .mac account on all 4 which is now upgraded to the MobileMe Service.
On 3 of the computers it's working fine but computer #4, the Mac Mini is where I am having the problem. I have each computer set up for Syncing of Data and both screen sharing and file sharing. On the Mac Mini I'm only interested in setting up Screen Sharing and am not interested in setting up File Sharing or Data Syncing.
For some reason it simply won't work with Back to My Mac. I thought it was a general MobileMe issue so as a test I set it up to sync a few items and it went off without a hitch and obviously it is set up and logging into my MobileMe account.
What happens is when I go to the Back to My Mac screen it immediately has a yellow circle and says Connecting. It just stays on this screen and sometime never goes further. Quite a few times it does though and it will say Back to My Mac: On and still have the yellow button but instead underneath it says with a Yellow triangle and exclamation mark "Back to my Mac isn't working properly because there's a problem with your connection to MobileMe" Since syncing is working fine I don't see how there can be a connection problem.
I have tried rebooting the computer and turning off Back to My Mac and turning it back on but the same happens again. I have even wondered if this could be some type of firewall issue. Any ideas of what could be blocking Back to my Mac from connecting?
Screen Sharing needs a minimum of 300k upload speed so maybe the other Macs are saturating the line?
Can you disable Bac to My Mac on the other machines and then try it on the Mini as a test?
I'll give that a try. I'm not sure if this is related in any way but the 2 MBP's and the Mac Pro are all using WiFi through a Wireless N Airport Base Station. The Mac Mini is on wired but is on Gigabit ethernet. Also, just to set it up, etc. it's all being done in house and I haven't tried to use it remotely. I'm going to give that a try though and see if that fixes it.
I recently had the same problem and it was my router that wasn't working properly with UPnP. A reboot of the router took care of it. Make sure you're using the latest firmware on your router, too.