The only thing I could see was this:
The problem was not concerned all databases, but at least the problem was consistent in multiple NSF’s. Building and refreshing the project no effect. Also removing the applications from the workspaces, restarting DDE and re-opening the NSF’s did not change the editor’s behavior.
At this point I was getting a bit annoyed. Colleagues reported similar problems in the past but none had a suggestion other than re-installing the client.
So I let my stubbornness win and installed Feature Pack 9 instead. After 10 minutes (!) the installation was complete so I started DDE and opened the unwilling script library. Hurraaaah! The problem was solved, I could see the code in the editor.
However I noticed some missing in DDE: the nice Swiper 2.0.1 toolbar buttons where gone! So I removed the files from the notes/data/workspace/applications folder, downloaded Swiper from OpenNTF again and installed them. Back to normal. Well not quiet yet.
When working on an XPage I noticed that all the plugins I was using where not recognized anymore. For example the Debug Toolbar plugin. Again, I could not see the plugin disabled under Application Management. So I removed the files, downloaded the plugin from OpenNTF and re-installed it. Now the plugin was visible again in DDE.
So the feeling I am left with is that this Superhuman Software is not so Superhuman anymore.