testing without plugins produces internal server error

https://www.arts.uk.net is a fairly simple subdomain multisite.

Troubleshooting a problem with shortcodes and timthumb I just deactivated all plugins, expecting the problem to disappear, but what disappeared was the whole front end. The white screen of death surely should not come up like this?

Could someone give me a steer as to where to look. The only thing I can think of is that I, very carefully, dropped all the database tables that had been created by buddypress, which was tested and abandoned. But I am sure I didn’t get rid of anything other than bp tables.