I'm not experienced with these plants, Hetty, but my guess would be a blight or other fungal disease. I noticed that there are water droplets on the leaves...is there any way to help the leaves stay drier to retard the spread?
I don't use fungicides, so I can't help with types or applications to use, sorry.
If it were my plant, I'd probably quarantine it, if possible, and then cut away the infected leaves, dipping shears or knife in a disinfecting (10% bleach and water) solution after each cut and discard in the household trash. Then attempt to keep the leaves as dry as possible, while allowing the plant to have as much direct sun as can be had.
Hopefully, someone with more experience will chime in soon, but I wouldn't wait too long to contain it, especially if you have others of this type of plant.
Have you noticed any bugs present?
