Please have a look at the following content:-
K2 blackpearl Server service - Error Invoking 'StartHostServer' : The type initializer for '<Module>' threw an exception. Stopped Hosting Server Runtime:
K2 Configuration Service error when starting K2 Server in console mode: "Invoking 'StartHostServer': SourceCode.HostServerLib: Object reference not set to an instance of an object":
Issue with Starting the host server services
K2 Server service fails to start after upgrade to 5.3 when K2 is configured to use SQL Server alias
K2 Service not starting after new install
Receiving error 2001 Error Starting Host Server: Object reference not set to an instance of an object after enabling SmOs services endpoints:
All the best.