Well, in my amateurish opinion - there was nothing wrong with the existing AES encryption implementation. But they also had a key derivation function based on running many iterations of AES, which was weird and non-standard, I mean it seemed like it would work, but it's not proven in the cryptographic community. Now they've switched to Argon2 which is a very well reviewed KDF and I have a lot more faith in that.

Code for the old KDF is here if anyone's interested: https://github.com/keepassxreboot/keepassxc/blob/7a55ab64d83...