Earlier this week, there was a bug introduced into the Salesforce Spring ’12 release. Unfortunately, the result of this bug causes some of the newer versions of LOOP (versions 5.5.0 – 126.96.36.199) to return a blank page instead of the normal LOOPlus DDP selection page (or will not auto run if it is enabled on your button). Currently, this bug is limited to Salesforce sandboxes that have been upgraded to Spring ’12. However, the Spring update is scheduled for this weekend. Any accounts experiencing this issue will need to upgrade with the patch now available on the AppExchange that includes a work-around for this bug.