I'm going to mention something that I'm surprised no one else picked up, the error the plugin is encountering is HandleError 1 (
HandleError_Changed, which stands for the Handle has been freed and reassigned)
It should be noted that upon a SM plugin timer's completion, it destroys it's own handle rendering it invalid to delete manually (hence the error), to fix the error, set the variable holding the timer to null BEFORE returning Plugin_Stop (or Plugin_Handled?)