login after sleep fails "incorrect password", select 'switch user' login with same password works

often after waking laptop from sleep mode inputting login password to password field on login window fails with message "incorrect password".
Sometimes re-entering password will work, other times not after many attempts.
Selecting 'change/switch user account' and re-entering the same password for the same user at the login screen will work.
There is only one user account on the machine.
This behavior has only happened recently but persistently.
On the most recent wake attempt the login window did not appear at all and I had to hard shutdown and reboot in order to log in.
There is no bug report of this issue for this kernal [4.15.0.70]
running Linux Mint 18.3, MATE 64