What type of encryption has been used to Zyxel VMG8924-B10A config file?

[chants]

Looks for sure to be base64 but it's of raw byte data which could be a 16 byte hash and salt of the password or what have you. You should have mentioned the length of the decrypted data which looks like 43x6//8=32 bytes. This is where the firmware comes in useful. The exact version is best but if the manufacturer website has one and you think the encryption and password storage did not change between the versions then you could study that one for clues at least. Otherwise desolder is best option.