I was also having the same exact problem as you guys are having. Then I tried this suggestion out and it worked perfectly for me!!! To explain what to do step-by-step, first you open up a personal hotspot on your phone and let your computer connect to it. For example let's say my phone's personal hotspot name is "Royals Wifi." I go to my computer and connect to "Royals Wifi" instead of my usual wifi. Then I open up the MapleRoyals client and log onto an account that had this problem with the PIN system. Then I register a PIN and it allows me to log in. Then I can log out, turn off my personal hotspot, connect back to my normal wifi, and everything is resolved!! To add to what @Shiyui was saying, I think the problem does indeed have something to do with the previous server check in which you cannot get past the login screen of an account that you did not register a PIN before the last server check IF you are using the same IP address that you use for your main accounts. Please correct me if I am wrong, but that may explain why connecting to a personal hotspot resolves this problem.
If changing the login IP address fixes the issue, users should be able to use a VPN instead of a personal hotspot. Just need to find a VPN IP address that isn't banned. XD
Same problem. Just redownloaded the game, haven't played in a while, and was prompted with the pin registration that dcs me right after the input .