In the earlier versions of SalesLogix Web 7.5 the Sdata deployment used the SalesLogix application pool. But since 7.5.3 Sage has used a separate application pool for Sdata. I would recommend adding a separate application pool for Sdata for anyone on version 7.5.1 or 7.5.2. Here are the steps to adding the App Pool. In IIS 7.x open the existing Application Pools and highlight the SalesLogix Pool. Right mouse click and select Add Application Pool. Name the pool Sdata and select .Net Framework v2.0.50727 version and Classic for the managed pipeline mode. Leave the start application pool immediately checked. Once this has completed your app pool will show up in the listings, once again, right mouse click and go to Advanced Settings. Check the identity and make sure it is pointing to the Webdll user. After verifying the user you are done. Your next step is a rebuild and redeploy of the Sdata portal using the new Sdata Application Pool.