The second challenge in the "Encoding" category is called "Baseic".
This challenge requires another decode with the provided data:
Funny how the challenge-name, when said out-loud is saying "Base I see" - almost telling us to think in some sort of Base-encoding.
The string provided very much assembles some kind of base-encoding, as we could derive from the challenge-name, due to the alphanumeric characters without any special characters or other weird stuff. A quick base64 decode didn't give much, so off to https://gchq.github.io/CyberChef/, a site very intuitive for quickly trying different decoders. And behold, base58 seems to be the one we are looking for. This could of course be written in python to extract the same information.
#! /usr/bin/env python3
import base58 # pip3 install base58
data = "4YX1Cb6Q7vDq5bt3SiYsnCFq4Sp5bBbkYTdiFgtyCUkGhPeM5pVJy4g"
With that, we have the next flag and another 200 points to the scoreboard.