I had the same problem yesterday when my Galaxy S4 updated its operating system. I was temporarily locked out by Wheres My Droid and when I entered my usual pin the program emailed and SMS texted my husbands phone that my pin had just been entered correctly. It then locked me out permanently!!!! (Go figure!!) The only way I could get back into my phone was to uninstall the programme and then reinstall. I think Find my Droid has a problem with the new software update. Great piece of software though, I have located my lost phone several times using this program so was determined to keep it. So my advice
uninstall and reinstall and see if that works for you, good luck