I remember this problem. It also didn't let me chat, so this is what I did.
Before this, I installed the Client Side GUI Editor packaged by Space Guy.
Then in game :
1 : I upload my file.
2 : I can't chat or use my tools. Uh oh.
3 : I think that the gui window showing the loading progress that just popped up is overlapping the PlayGui, so that's why.
4 : With the GUI Editor installed, I press F10 and then press F10 again.
5 : Problem solved. Behind the scenes, my file is uploading, but I don't have that progress bar that prevents me from playing.
Also if you are hosting the dedicated server, you may rename your .bls save file to "temp.bls" and place it in the server's base/server/temp/ folder then in-game type /reloadbricks to speed things up.