The Photosynth servers have had some trouble this past week.
( see this entry in the Photosynth blog for more details: http://blogs.msdn.com/photosynth/archive/2010/04/14/power-failure.aspx )
and that may possibly be what is responsible for you seeing this message.
I have, however, just uploaded a small test synth successfully, so it may simply be a problem with your internet connection configuration.
If you have tried *temporarily* disabling any firewall that is protecting your home network to allow the synther to talk to the website and this problem doesn't go away for you, please email Tony Ernst ( TonyErn@microsoft.com ).