White Screen of Death on Cron, fixed with update.php One installation of Drupal has been fine for a long time, but has started developing a WSOD on most (maybe all) cron invocations.

Looking through Drupal's log messages, I see a few normal cron messages, and then this:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in theme_get_registry() (line 277 of theme.inc).
Warning: array_keys() expects parameter 1 to be array, null given in ThemeRegistry-&gt;initializeRegistry() (line 413 of theme.inc).
Warning: array_fill_keys() expects parameter 1 to be array, null given in ThemeRegistry-&gt;initializeRegistry() (line 413 of theme.inc)
Warning: PHP Shutdown: Argument #1 is not an array in ThemeRegistry-&gt;set() (line 457 of theme.inc).


I then see the following repeated many hundreds of times, once for each theme hook, it seems:

Theme &lt;hook name&gt; not found. Warning: array_key_exists() expects
parameter 2 to be array, null given in ThemeRegistry-&gt;offsetExists()
(line 421 of theme.inc).


So that is repeated a few hundred times, where &lt;hook name&gt; is every theme hook in the entire Drupal installation.

It has been running a custom theme, but that theme has behaved fine for ages. In fact, I have not made many recent site changes. I've only updated Drupal to 7.52.

Any ideas how to proceed?
Looks like an issue with either a theme hook implementation, or call to a theme function. In particular seems related to theme_block

While it could be something else, I'd suggest making sure you have the file block.tpl.php in your base and sub-theme directories and then clear cache.

Other possibilities are a file permission issue, make sure the files are readable by the webserver