Blank Index Page Problem

We’re using 2.5 and some of the sites are coming up with blank index pages.

We can access the admin control panel, and are also able to use the default and classic themes, but other sites using some other themes seem to having the problem. On two of the sites, the blank page appeared shortly after users edited a page and saved their changes. On another site, the blank page just appeared without any changes being made on the site or the server.

I’m thinking of two possible issues causing this:

1. Some type of database issue.
2. Something that’s being uploaded to the sites that is causing a conflict with the theme, such as a flash or shockwave file, or video file.

I’ve tried deactivatng plugins and that made no difference. I examined the index.php files and compared them to others on the working sites – they looked ok.

I welcome any ideas as to what is causing this and how to fix it.