Hello there! as you can see in this link, a small portion of players has always been relying on MapleRoyalsNoWindowedMode.exe Client to play the game. https://royals.ms/forum/threads/unspecified-error-fix.97969/ It was the only way we could get around the "unspecified error" issue and it worked fine for us with all the previous updatees until New Source Update #50 was released. Update #51 has the same problem too. it's still showing the same error message.
I have this same issue too. Can't play the game at all for now ): I hope it'll get resolved soon with an updated exe!