I am also having the same problem with a DTS package with an FTP step.

I recreated the package with all the objects minus the FTP. Copying all the relevant FTP configuration information into a text box within the package.

Then saved the package out as a structured storage file (*.dts).

You could also just use the Save As and backup the file.

With the backup safely stored I then add the FTP step and run the package. Next week when I go to run the package again it will give me the Unspecified Error - Cannot be opened.

Recreate and re-run, I haven't been able to find a true solution.