Been happening to me after server restarts. For now I am using TimedExecute to reload the plugin after the restart and that seems to work.
[Server Armour] User not checked. Server armour is not loaded.
Might have something to do with this message I sometimes see after restarts
Web request produced exception (Url: https://serverarmour.com/api/v1/plugin/status) (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Core.Libraries.WebRequests+WebRequest.<WaitForResponse>b__44_0 (System.IAsyncResult res) [0x00000] in <15f61ddda771464d8246ebdce8ff4811>:0