I think that's DrenDran's Blockland Essentials
Right, so you have a mod installed that attempts to replace every string in the game. I'll be on my way now.
Too bad I didn't catch this before.
The Welcome %1 text was fixed a while ago. It was never meant to be a tagged string.
I might make the errors stop showing up in the console completly, since it really isn't a bad thing, but I guess it scares people anyway.