Trying to open an old bitcoin core wallet.dat file fails I am trying to open an old (~2012) bitcoin core wallet.dat with a bitcoin.com wallet, and it says that my password is incorrect, although it works great on my bitcoin core wallet.

Was there any change in the protocol since 2012 that could explain such a problem? (If I understood correctly, the only wallet in 2012 was bitcoin core's)
There are probably other ways, but I tried this a while back with an old wallet.dat from ~2013 and it worked. You can download the Bitcoin Cash release of Bitcoin Unlimited and open that wallet. Bitcoin Unlimited is the reference implementation of the Bitcoin Cash client that is a fork of Bitcoin core so it works with those wallets. To do any transactions you'll have to let it download and verify the blockchain and that takes forever, but it will work.

Once you have it synced, you'll see your balance (assuming you had a Bitcoin/BTC balance before the fork) and you can then transfer your Bitcoin Cash (BCH) to another BCH wallet or exchange (e.g. Coinbase/GDAX is a simple way to exchange now).