at MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery

Unfortunately, Oxide uses an old library for connecting to MySQL, MySql.Data.dll version 6.9.5.0, which was released on November 26, 2014. Because of this, some SQL commands may cause errors, in the context of this plugin, this happens when INSERT IGNORE queries are executed.

This error can be ignored since the query itself is executed and the data is successfully added to the database.

However, if you wish and have access to replacing MySql.Data.dll(*SERVER*\RustDedicated_Data\Managed\MySql.Data.dll), you can update this library to a newer version from the official Oracle website: https://dev.mysql.com/downloads/connector/net/ . Make sure to check your OS version when downloading the file.
After installing the downloaded .msi file, you will find the MySql.Data.dll file in the directory where you installed it(by default: "C:\Program Files (x86)\MySQL\MySQL Connector NET 9.2\"). Simply replace the existing MySql.Data.dll on your server with this one. Note, the server must be turned off.
Proceed at your own risk, I have tested this in the context of this plugin and everything worked fine. Therefore, I recommend using the API method instead of MySQL.

MySql handle raised an exception (NullReferenceException: Object reference not set to an instance of an object)
  at MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery (System.IAsyncResult asyncResult) [0x00022] in <8d7380b3d3cf4bd8be324c584f371b44>:0
  at (wrapper remoting-invoke-with-check) MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery(System.IAsyncResult)
  at Oxide.Core.MySql.Libraries.MySql+MySqlQuery.Handle () [0x000f2] in <021a1dba6a754e42a6cc7771fdfee190>:0

Changed Status from Pending to Not a Bug

If you encounter a similar error, you can either create a separate topic or post the error log here.

Changed Status from Pending to Not a Bug

Changed Status from Pending to Not a Bug