maintenance mode -1, white screen

I was trying to update plugins from the update screen in the normal way and the process started, but immediately got stuck. It says:

Enabling maintenance mode…
-1

Clicking to anything after that produces an empty white screen. The only way to get the site back up is to delete the .maintenance file in the root directory. I tried updating plugins one at a time but the same thing happens. I also tried enabling maintenance mode using the WP Maintenance Mode plugin first, but the same thing still happens.

Any ideas?
Thanks

The page I need help with: [log in to see the link]