I’d guess Botrytis as it will take down a branch in just a few days. And especially since it’s effecting a specific branch(es), as opposed to the entire plant. Look at the effected stems where it meets the main stalk and see if there is a gray fuzzy mold. If you do have Botrytis the effected stems are toast, you’ll have to cut them off and toss and hope that you can keep it from spreading.
Have to be careful on soil mix. I had one bag that was miss mixed from my usual mix. Noticed the plant wasn't doing as well as others. Came time to transplant to a larger container and the bottom half of the soil was waterlogged and stunk. I noticed that it had very little aeration mix and was almost just mud.