Yes, I would like to know too. Maybe try to setup a new clean installation in the other directory. Clone database. Copy /data dir. And see what happens? There may be a lot of reasons of this. But first is to try get some error report.
I remember that some time ago I've got similar issue and it was related to the APC cache. The fastest solution for me was to "touch" (change modification time) of each file. Then it started working.
Two days ago I was setting up my own installation to work with PostgreSQL. We've got some undocumented feature to run more installations in the same directory, but with different data dirs (so different config, database, etc.). It was stuck on the blank screen too. I did some syntax error in the map.php file (that is responsible for mapping data dirs to domains). That was the issue, but I couldn't find it, because map.php was included with supressing errors :/
