I've had trouble decrypting notes a few over the past year or so. I recently had this problem which lead me to this thread. I'm was running Mac 6.1 but another thread suggested i upgrade to the 6.11 beta which might fix the problem. It did not. I was able to decrypt the note from my iphone and then re-encrypt it from my Mac 6.11. I did have to leave the content exposed for a minute but I'm glad I was able to get to my data.
This seems to be a recurring problem with Evernote or a problem that they are not able to fix. If I find another product that will let me encrypt my data, I all for jumping ship.