I made some changes, and now I can open Prolog files in Eclipse without this error occuring.

What I did:
I restarted Eclipse.
I right-clicked on the project containing the source file hello.pl. In the context menu, I selected ProDT... Add Prolog Capabilities.
Then I double-clicked on the file hello.pl in Eclipse's Script Explorer tab.
The source file now opened in the editor.

I don't quite know what helped, but something certainly seemed to help.