This is really weird. About half the time at random, when I try to install something in the Software Center, it fails to accept the password (which I carefully typed but failed repeatedly). I then give up and just click "Install" or "Remove", when then miraculously works anyway.
It happened again just recently, but unfortunately it's not consistent so I can't be sure of the answer to your question. If you like we can leave this open for a while and I'll report back if it recurs.