Invalid reg code

I’ve recently installed gsb on a second pc, input the reg code and had that accepted and been posting challenges to myself with no problems. Now, going back to the pc I initially installed the game on after quite some time, when trying to post a challenge, all I get is an invalid reg code error. Also, I am unable to download any challenges (from anybody) as they all come up saying unavailable and they’ve been removed from the server. The latest challenge I can see was from Feb 25th.

Does having the game on a second pc (with a different username as well) make a difference, and is there a way around this?