I have seen this issue come up twice now.  Basically what happens is any customer running i2 on a Mac cannot connect to i2 server.  Its specific to a Mac OS, it was happening on Corel and Adobe installs and problem showed up here on the Mac.  Ruth had the problem, Emily also confirmed it and Ruths customers had the problem.  What happens is you load Core/Adobe, login (doesn't matter what server) and you get the error "Cannot connect to Server".

Yes if you run i2 on a Windows pc you can connect fine.  The resolution to this was Alberto had to restart the i2 server.