There's currently a problem with a blacklist server which is used by Team17's WormNET. With that one server being "down", the blacklist check cannot be done, resulting in a "negative" response and a ban. Long story short, you're not the only one having this problem. Just hope it'd fix itself in a few days.

Meanwhile, you can try using an alternate WormNET server. To do that, start W:A, then go to Options, then Advanced, and enable the "Use community server list" option, then go to online play like you would do normally. Instead of throwing you right at Team17's server, it'll give you an option to choose a server yourself. I'd recommend "StepS' server". Note that there's no HostingBuddy on this server, so you gotta host games yourself (or using WormNAT2).