I had the same problem when I patched to 1.5. Whenever I selected one of my saved games or ANY of the factions for an imperial campaign, I was booted back to the menu.

I uninstalled RTW, reinstalled, patched to 1.3, patched to 1.5, and then re-edited the text file to make all factions playable. After that I have been fine. I am able to access my old saved games as well- they were not deleted in the uninstall.