CL_BSP_PAGE_BASE does not exist

Hi all, I just upgraded my CRM application from SP44 to 53 (ABA and BASIS). Before the upgrade all standard PC-UI applications worked but not anymore. The problem is class 'CL_BSP_PAGE_BASE' is not available anymore. In my test system it does exist but in my development system, after upgrade, not anymore. Anyone any suggestions? Tiest.