I have even tried deleting the tmp and cache folder from manage server but nothing worked..
Unless you're running out of CPU, disk space or other resources, having WebCenter Sites and the database on the same VM shouldn't be any problem. It may affect performance but it shouldn't cause any errors.
Are you getting anything in the logs when you click on the Asset type? If not, taking a couple of thread dumps and comparing them in ThreadLogic or a similar tool may help determine what's going on.