Addon map consistency string error: Either you or the server has an outdated version of the map. Update your map with the latest version and make sure to remove the old version from your addons folder, but if theirs is outdated, you can't do anything.
Official map consistency string error: Most likely your files are corrupt and you need to validate your files.
Other game files when connecting to official dedicated servers: Your files are corrupt and you need to validate.
Other game files when connecting to locally hosted servers: Most likely the host did not turn off consistency, tell them to go back to lobby and enter this in the console
Consistency is enabled by default in locally hosted servers, but disabled in official dedicated servers. It means that the server is going to force you to have the same exact files as it does, but applies to mods only; if you don't have any mods installed that alter the file you're seeing the consistency error for, it is not the server's fault and your game files are corrupt.
If you've validated your files and you're still getting a consistency error on servers with consistency off, check your mods list. Mods with reputable authors like Rayman are generally safe to use and won't mess up your game files, and since l4dmaps is moderated its mods are generally safe. Some mods, however, especially mods from the workshop from authors who are not careful, will screw up your game; all you can do is unsubscribe and remove, and leave a comment on the game page.