Looks like a bug in the forum software. If it’s open source, look through their GitHub (or whatever) issues to see if it’s a known bug and see if anyone’s working on fixing it. Then wait for them to update their forum software and try again.

Edit: actually, this error indicates they’re not using parameter binding to build queries and aren’t even properly escaping input. Assume the database is already compromised and don’t sign up or post on this forum. Whomever is running it doesn’t know what they’re doing.

If you already signed up and you used a password that you use anywhere else, change it on any other websites that share the same password.