Occasionally MBP won't unlock after sleep, until restart.

Occasionally, but enough to annoy, my company provided 2015 15" MBP with Latest El Capitan installed, won't unlock after waking from sleep.

As usual I would open the lid and type the password, but the screen would freeze for like more than a minute, then jiggles the password textbox.

I tried the switch user option and entering different user names and passwords configured, all the same response.

When I try the restart option, it warns me other users logged in, so asks me for a username and password in a separate dialog box, and when I enter the username and password there, it says "Enter valid username and password" or something similar.

It's worth to note that near to the username textbox, there is a small text that says "Network users unavailable", although I am using a username that's on the mac and not on network. And my WiFi remains connected all the time.

I have to press and hold the power button for a cold boot everytime this happens to login into mac.

I couldn't find any other posts mentioning this issue. Is mine a one of case ?