What Cubase version are you using ? I guess that it’s a recent one. At first glance, I would try to install SX3, then load and save the project again with it. Normally, it then should be usable by all the Cubase versions that were published after : see here, for more details…
But what is strange, though, is that normally a SX1 .cpr project file should be directly compatible with SX3 and further versions, contrarily to an .arr one. So, still something to try, I think, but without being sure of a successful result…