We had not seen this error in the InforCRM Customer Portal until recently, and the befuddling aspect was the SLXClient attachment upload worked perfectly. The Application Architect setup was checked and IIS was setup and running with the WebDll user running the CustomerPortal application and web pool. So where is the issue?
Well apparently in IIS 8.5, the WebDav Module can override security, and restrict the system from allowing attachment uploads. We removed the WebDav module from IIS under the Customer Portal application and ran an IISRESET. Removing the WebDav Module can be done on a dedicated InforCRM web host only.
I hope you don’t run into this, but if you do, this solution will save you!