Bitcoin QT wallet crashed. Recovered using Bitcoin Core. Now password rejected! My encrypted Bitcoin QT wallet crashed years ago but I have backup copies.
It was an earlyish version of Bitcoin QT wallet, first transaction on it is 09/2012. 
I have loaded the backup into Bitcoin Core v0.14.1 (64-bit)
Full transaction history is there, wallet has synced. 
The password is rejected.
I wrote out the password exactly for safe keeping so I know it is correct and never changed it. There are a number of send transactions so it seems I must have used the passphrase a few times.
Have entered in every imaginable variation.
Could it be because of the upgrade from QT to Core?
As far as I know the versions should be backward compatible but I don't know how far back, 5 years is quite a long time, Bitcoin Core have had 30 releases since then.

I found two changes that might impact wallet compatibility in my opinion:

Version 0.9.0 (19 March 2014):


  Also, the first time you run a 0.8.x release on a 0.9 wallet it will
  rescan the blockchain for missing spent coins, which will take a long
  time (tens of minutes on a typical machine).


Version 0.13.0 (23 August 2016):


  HD wallets are incompatible with older versions of Bitcoin Core.


Anyways, I would suggest to make a backup copy of your wallet, download an older version of Bitcoin Core, copy the wallet to the data directory of that client (you might need to specify it using -datadir C:\bitcoin_old\data), start the client (no need to synchronize the blockchain), export the private keys, exit, move away the wallet.dat from the data dir of your new Bitcoin Core client, then start it and import the private keys to your newly created wallet.