I noticed you said you were a crab newbie... does the other RCC have land access? I don't mean to imply thats what is caused you to lose the crab or that you are well informed on proper care but if you have another crab in there you may want to take preventative measures.
Again, don't know you or your situation. Bad molt seems like a reasonable yet unfortunate cause.
(I killed a couple crabs after I keeping fish for a couple years as I had done minimal reading and only saw what I wanted to see. End of rant/confession)