If you don't have a wallet password set you can simple open the wallet file in a text editor and you will see the seed words in the beginning. Import them in another non-BIP44 wallet (e.g. electrum). The new Bisq version is not compatible with the old seed words as we changed to BIP44 wallet structure.
Another user [2] claims that you can restore the seed in Electrum. I personally never tried neither of these solutions, but they can probably help you in some troubleshooting.
Thanks a lot for your help! Actually, I don't have any idea if I've ever deposited coins to this app. Tried to import the seed to Electrum (BIP39, Legacy) with path m/0' as well as m/44'/0'/0' but don't see any funds nor transactions.
In November 2016 I was still using MultiBit Classic – even though I've already had Electrum running – and I've played around with other wallet apps. I think I remember how I've traded some ETH with Bitsquare but I'm not 100% sure. But let's say I have bought ETH with Bitsquare: what happened to them? Is there a way to recover the altcoin wallets as well from Bitsquare?