Exactly the same just happened to me, while I was using the tablet it locked and would not unlock with the PIN, saying try again in 30s.
Even tried the findmymobile service and it could connect to the tablet, make it ring, back it up but NOT unlock it. Just came up with an error when you tried.
Finally found how to force power off the tablet (normal power off and restart and safe mode options just asked for the PIN again).
To force power off, press the power and volume down buttons together for more than 7 seconds. It then restarted and asked the PIN which worked as normal.
... View more