[Plugin: WP Super Cache] "Buffer is blank" but no other plugins are running.

After caching was working previously, noticed that caching wasn’t happening now and started to debug.

Using WordPress 3.04, PHP 5.2.6 with WP Super Cache 0.9.9.8. All other plugins are disabled and using default Twenty Ten 1.1 theme but for some reason caching still isn’t happening. Have tried mod_rewrite, PHP and legacy. None work:

05:12:11 / No wp-cache file exists. Must generate a new one.
05:12:11 / In WP Cache Phase 2
05:12:11 / Setting up WordPress actions
05:12:11 / Created output buffer
05:12:11 / supercache dir: [CUT]
05:12:11 / Output buffer callback
05:12:11 / Buffer is blank. Output buffer may have been corrupted by another plugin or this is a redirected URL. Look for text 'ob_start' in the files of your plugins directory.
05:12:11 / No closing html tag. Not caching.