Constant warnings in logs - SendMessage failed: InvalidParam

Every time a player disconnects from my server the logs look like this, always with exactly 3 'InvalidParam' messages. Is anyone else getting this? I'm wondering if it's the server or one of my plugins causing it. There don't seem to be any ill effects from it so far but I'm curious.

*.*.*.*:00000/76561############/JohnDoe disconnecting: closing
*.*.*.*:00000/76561############/JohnDoe SendMessage failed: InvalidParam 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 35)

*.*.*.*:00000/76561############/JohnDoe SendMessage failed: InvalidParam 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 35)

*.*.*.*:00000/76561############/JohnDoe SendMessage failed: InvalidParam 
(Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 35)

post plugin list.

Thanks for the help - it seems that SignArtist is the culprit.