I am not sure what this is folks, so far, I haven't seen this reported and if there was an error in the Warn.php file, it would affect everyone (unless KirbyDE has changed something). So, all I can suggest is that you check your postbit template edits.
When you move your cursor over the Warn XYZ button (or link) at the bottom of your browser window, it should show you the command it will execute, something like:
Code:
http://www.myserver.com/Warn.php?do=WarnUser&id=5420&post=61700
Do you see a number next to the id= part?