I could export and import a campaign without a problem.
But there is a known bug I fixed yesterday.
I am about to publish it to production (the regular site).
Do your pages contain any 5RD templates?
If yes, please try Ctrl+F5 the Preview site in your browser (on Windows).
In the lower right you should see:
If you see this, please try Clear Cache and Reload All:
Are your pages now visible?