It sounds like you're experiencing the same issue described in this thread: https://wurstforum.net/d/21-chat-disabled-due-to-missing-profile-public-key.
While it may seem like a bug, it's actually a security feature to protect you from Mojang's draconian chat reporting system. The server in question is trying to force you to send chat messages that can be reported to Mojang (also known as "signed messages"), which is something Wurst doesn't allow by default to keep your account safe.
If you are comfortable with the risks involved, you can re-enable reportable messages by turning off the NoChatReports feature in Wurst. If you have the latest version, you'll find it under the "Chat" category in ClickGUI. Just toggle it off.
If you prefer to use the Navigator GUI or you're using an older version of Wurst, you'll find it by opening Navigator and searching for "NoChatReports".
However, do note that by disabling NoChatReports, you run the risk of being chat-reported and potentially banned by Mojang. See this FAQ for more details on the risks of chat reporting.
I hope this helps! Let me know if anything is unclear.