Cause was probably the choice of soil. I dislike the ''moisture control'' for a number of reasons, one being it may stay too wet. Moisture control soil is for pots where you can't water them more than once every week or two, or even less often.
Try to let it get almost dry before watering again, and cross your fingers. If it starts to keep leaves, it might be okay. If new leaves keep wilting and dying, it may be impossible to save.
If it was kept too wet, the root tips die, the dead root tips become the entry point for fungal and bacterial issues. Once this cycle starts, the Serissa will be very difficult to save. Using a fungicide or bactericidal treatment is unlikely to work well enough to save the tree. Serissa are less expensive than the chemicals. Just start over if you have too.
An inexpensive topical sterilant is 3% hydrogen peroxide from your local pharmacy. I've used it straight out of the bottle for sterilizing wounds or affected leaves and or stems. Dilute the hydrogen peroxide 50:50 water

eroxide and then water the soil of the tree. It will foam, just let it do so, let drain. You do not need to flush with water afterwards, the hydrogen peroxide will break down into oxygen and water in less than 30 minutes after contacting the soil. Once the foaming stops it is pretty much totally broken down. It might help.