You should stick with the naming conventions and call it Client_Underground, not GUI_Underground.
Well, GUI actually is a previously used prefix, even if not as common as Client. Yet, you don't really see the name of the add-on, except on the add-ons folder. Does it make a big difference? I mean, I can rename it if it's an issue, just curious why.
Inventory system is broken as forget, it claimed I cheated repeatedly if I didn't check my inventory once on a while
Make sure people can only pick up 1 brick per second if you don't want inventory editing
So bugs pop up in alpha stages. It will be addressed and fixed when I next work on it. It was working for the majority of the players, it looks like something weird happened in your instance, we didn't host the first alpha stage with a completely broken inventory.
In other news, alpha server is now down, and it went well. We got some (many) bugs pointed out, great ideas were mentioned, and things were received fairly positively. Hopefully I can fix up those bugs and add some more features for tomorrow or the next couple days!
All in all, the alpha server today might have been a bit rushed, but it turned out alright, and I think hosting often will be a good strategy, as many bugs can pop up when you've got 8+ players doing things, as opposed to 1 or 2.