Why is a page blank, or half missing?
A blank page, or a page that seems to have half missing, is normally a sign that an error occurred. If (as is recommended on production systems) you have display of error messages turned off, it can be very hard to understand what is going on. The solution is to go to Admin -> Server -> Debugging (…/admin/settings.php?section=debugging) and set Debug messages to ALL, and turn on Display debug messages. Then go back to the problem page and you should see the error message. (If this is your production system, copy and paste the error message to a safe place, then immediately turn debugging off again.) If the page that is broken is the admin page, then it is a bit harder to fix. The solution then is to update the setting directly in the database.
A blank page, or a page that seems to have half missing, is normally a sign that an error occurred. If (as is recommended on production systems) you have display of error messages turned off, it can be very hard to understand what is going on. The solution is to turn on the debugging options temporarily. Then, when you go back to the problem page, you should see the error message. Once you had finished solving the problem, remember to turn debugging off again.
