Encountered this error, while trying to debug my other issue(NpgsqlConnection hangs for hours on opening connection).
When I provide SSL Mode parameter of Require in connection string, I get the following exception: Npgsql.PostgresException: 42704: unrecognized configuration parameter "ssl_renegotiation_limit"
While looking online for possible solution, I found this thread created by Npgsql nuget package owner. And from what it looks like, it may be an issue, that's in a feature request state: https://forums.aws.amazon.com/thread.jspa?threadID=229990
It's hard to know what's going on because you're not including any details on your PostreSQL version. However, All recent versions of PostgreSQL recognize (and ignore) ssl_renegotiation_limit. You're either using a very old version of PostgreSQL, or possibly Amazon Redshift, in which case you need to add Server Compatibility Mode=Redshift as specified in the Npgsql docs.