You're not the only one. I tested it myself and it looks like the official agar.io client is no longer compatible with the server. You could try finding a different server, or just use a different agar.io client, i.e.
There's an Issue in the server's GitHub repo about this:
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It contains links to various unofficial agar.io clients. They may not be completely safe (be careful of random websites etc etc), so visit them at your own risk. At least one of them appeared to work for me with the server, but again, it's your risk. Make sure to append /?ip=127.0.0.1:443 to the URL.
You could also try finding a client on GitHub or elsewhere to host yourself.
Best of luck.