ArcGIS Server log messages numbered 100000 and higher originate from ArcObjects. The ArcObjects libraries that generate these messages are as follows:
Getting help for ArcObjects log codes
ArcObjects libraries sometimes have constants describing the server-related messages that they can return. These are contained in enumerations ending in ServerMessageCodeEnum. Examples are esriServerMessageCodeEnum in the esriSystem library, and esriWMSServerMessageCodeEnum in the Carto library. The ArcObjects library reference topics for these enumerations may contain additional information about the messages.