After an upgrade from drupal 5 to 5.3 this weekend I ran into an error on a client's site - which wasn't hosted on one of our servers, so I couldn't look into their server error logs as I didn't have permission!

It was a pretty frustrating place to be as I couldn't tell why I was getting a blank white screen when I tried to access only certain node/add or node/edit pages - at the time I didn't realise that those particular node types used a certain taxonomy that was working with the form tweaker module to make term selections checkboxes instead of lists!!

Anyway, I found a tip that helped me out; add the following line to your settings.php file (in sites/default via ftp) et voila - server error messages will be printed on the not-so-blank white screen, which you can then use to trace the error - which for me was something funky with the form tweaker mod.