Why would my /admin/reports/dblog get "stuck" on only showing "page not found" errors?
Try having a look in your server error_log file that will probably tell you what's causing the problem. My guess is an unsupported character or something has been recorded in a log message.

You could just try clearing the actual log table in the database. Looks like you're on drupal 7 so you'd want to truncate the watchdog table to remove all logged errors