After developing a custom Web Service to be hosted in SharePoint (based on this HowTo article on MSDN) and deploying it on a testing environment, I got some entries in the EventLog over there, stating:
“Unable to connect publishing custom string handler for output caching. IIS Instance Id is ‘xxxxxxxxx’, url is ‘https://internalsharepointserver/somesubfolder/services.asmx“
The PublishingHttpModule trying to cache an unmanaged path seems to be the problem here. When enabled for a custom web application (such as my web service), it causes the eventlog entry each time certain requests like web service calls or static CSS files are made.