iMac log in issue (PW autofill problem)

Never had this before, only since installing El Capitan:

When I want to log in after Sleep, the usual log in screen appears with the correct user info and the blank password window.
Then, as soon as I touch any key, it auto fills the password window with a large number of dots (not my PW, which is shorter.
When I erase these, it does the same again as soon as I touch any key.
This goes on and on...
Only remedy I found is to power off the keyboard and then power it on again. Following this I can normally enter my password and log in.

Anyone else seen this? Suggestions?

Thanks,