Question: I set my username without spaces but it still says 'Server white-listed'. What should I do to join the server?

Answer: 
When a server is white-listed, it means that only the players on the white-list can access the server. This option is normally used if the owner of the server wants to limit which players can access their server.

The list of white-listed players can be found under `white-list.txt` in the root server directory. If you own the server, you can add yourself to this white-list to gain access to the server. (Please note that if you are an OP on the server you should already be white-listed).

If you are not the owner of the server, have a look at their website. Some servers require you to sign up on their website to be white-listed on the server.

If all else fails, contact the server owner directly.
Since Mojang supports name changes, whitelisted players and their UUIDs are stored in `whitelist.json`.  

If it's your server and you have access to the server console, use the command `whitelist add <YourMinecraftName>` or if you want to disable it, use `whitelist off`.  

If it's not your server, try to contact the server owner and ask him to put you on the whitelist.