Error codes 004, 010, 013, 015, 040, 050, and 072:
"These codes indicate a server error. The server may be inaccessible due to an update, bug, large download or network issues. The network status page may contain additional details."
Today, March 27, there is server maintenance being performed on ArenaNet's German datacenter, so any servers that run through that datacenter (most notably European servers) will experience network issues.