Thanks so much for your reply @ssprengel.
I checked everything you questioned and yes: the catalog is in the right place and the most recent one is the one that is opened (not one from a backup).
Now, I thought I had (almost) solved it: I used Time Machine to reset my Macbook to 5 or 6 days ago. And almost all the editing was back. That led me to believe that it was some temporary conflict.
This morning I opened LR5: and the same thing has happened again.. Most of the editing is gone, and LR cannot find some of the pictures (which I can manually retrace). By the way, this has happened to this summer's batch, my most recent editing work. The rest is not affected.. Still, this is giving me quite a headache..
Oh, I should add: the catalog is stored on my Macbook, but in the Dropbox folder. I'm going to try and find out if there is a Dropbox-synchronizing problem..