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 ‘http://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.

The quick solution is to simply remove the PublishingHttpModule for the custom web application. You can do so by editing the web.config file:
<httpModules>
    <remove name=PublishingHttpModule/>
    …
</httpModules>

To completely switch back to all default ASP.NET modules, use these settings in the web.config file:
<httpModules>
    <clear/>
    <add name=OutputCachetype=System.Web.Caching.OutputCacheModule/>
    <
add name=Sessiontype=System.Web.SessionState.SessionStateModule/>
    <add name=WindowsAuthenticationtype=System.Web.Security.WindowsAuthenticationModule/>
    <add name=FormsAuthenticationtype=System.Web.Security.FormsAuthenticationModule/>
    <add name=PassportAuthenticationtype=System.Web.Security.PassportAuthenticationModule/>
    <
add name=UrlAuthorizationtype=System.Web.Security.UrlAuthorizationModule/>
    <add name=FileAuthorizationtype=System.Web.Security.FileAuthorizationModule/> 
    …
</httpModules>