That sounds like a bug! The Continue Campaign button should be there :(
I can try debug it if you paste your logfile here please? (pastebin.com works well for this)
Mac: ~/Library/Logs/Free Lives/Second Earth/Player.log
Windows: C:\Users\username\AppData\LocalLow\Free Lives\Second Earth\Player.log
Linux: ~/.config/unity3d/Free Lives/Second Earth/Player.log