I found this in a blog a while ago but can’t find it now so here it is…
If you see this error in your event log the fix is fairly simple.
Event Type: Error
Event Source: Office SharePoint Server
Event Category: Publishing Cache
Event ID: 5785
Time: 6:19:13 AM
Unable to connect publishing custom string handler for output caching. IIS Instance Id is ‘461376054’, Url is ‘http://blahblah/page.aspx’.
The solution is to modify the web.config of the application. Remove the PublishingHttpModule module that was added by the root web.config:
<remove name=”PublishingHttpModule” />