Sorry you're having these problems, Cosima. It's heartbreaking to lose rabbit after rabbit, especially when you don't know why it keeps happening. It's hard to offer good advice because you live in Indonesia and few of us have much knowledge about keeping rabbits in your part of the world.
The problem could be a disease but it is more likely related to diet. It might help to keep an account of everything you feed the rabbits to see if a pattern emerges.
You could also try contacting the people you get your rabbits from and find out if they have experienced this in their rabbitry and at the same time see if they can give you some advice about what to feed the rabbits in your locale.
Also contact the vet who treats your rabbits for advice about what to feed the rabbits and also about remedies for the diarrhea that you could keep on hand.
I know this does not seem like much in the way of help, but it's all I have to suggest at the moment.
EDITED TO ADD:
It just occurred to me to google
raising rabbits in Indonesia. There is a lot of information there and I urge you to start reading it. Here's on link that should be helpful to start you off. These are top-off-the line rabbit authorities who I feel you can trust.
https://www.fao.org/3/U5700T/u5700T0d.htm
Good luck, Cosima. Wish there were more I could do.
~ Maggie