I am sorry for your loss .

It is hard to know for sure the cause without a necropsy. To be safe I'd recommend purchasing a new supplement and perhaps even think about rotating between 2 brands. Also be sure your calcium has vitamin D3 in it for proper calcium absorption. Do you use a multivitamin as well. Minute doses of other vitamins and minerals are needed for proper body functions, which can be found in an amphibian multivitamin powder. It could also be neurological, and/or both. It is difficult to know for sure.

As Caspian has mentioned, keep an eye on your other frog. Be sure he is acting normally and eating. They get used to having each other around and can become withdrawn after losing their buddy.