I met a very stange 'white page' problems. Following are some of my observations:

(1) When the problem occured, I was just browsing my site. Nothing change has been made to the site during the past few weeks.
(2) I am sure the hosting server is OK. I have another XOOPS site (same version 2.0.15) on the same server that works fine. Also I am not aware of any changes of PHP and MySQL version on the server.
(3) I modify the database entry to turn on the debug mode and I got black pages without any debugging information.
(4) I extracted the mainfile.php from a recent backup and upload that to the server - still no luck.
(5) I have checked all the file permissions and I am sure they are all correct. As I said I haven't changed anything on the site for weeks so this should not be a problem.

So I am running out of ideas. Any suggestion?