1. A Sandbox Tool, like Sandboxie or Avast's Sandbox/CyberCapture, could screw with the path directories and what it can access, leading into the client being unable to access the file, spitting out the 'Unable to Create Empty Document File.' When it doesn't, or it manages to reach the file and do it's work, a Sandboxing tool is basically a program that runs other programs under it's own, making it appear as if the program you ran was on a seperate device, simular to how a Virtual machine would, saving the files it leaves on a seperate folder on your computer.
2. Character Appearance URL is useless on the default clients as it does nothing, but basically you put your charapp link there and for clients that require charapps such as Novetus+ Clients (in #game-servers on the discord) will load that as it does. I cannot teach you how to make a charapp or how they work, because apparently a few admins think it's a funny idea to not find a temporary solution and just keep charapp guides and stuff a secret, but try and find a tutorial, and PLEASE for the love of god if you do find one, do NOT add any gears if you're planning to join other's servers.