The problem with Tetenal's BLIXes is that their BX2 part is essentially Ammonium Thiosulfate and Acetic Acid. Normally, if Thiosulfate oxidizes, it becomes more and more more acidic, until, below some pH threshold, it decomposes into Sulfite and Sulfur. With Acetic Acid already in the mix, it doesn't take much oxidation before that threshold pH is reached. That Sulfur is the precipitate you saw.

Depending on your willingness to home brew, you can replace that BX2 part with raw chemicals, or you just get a new kit.