I enabled logging using http://codex.wordpress.org/Debugging_in_WordPress#Example_wp-config.php_for_Debugging
and I get this message:
[07-Feb-2015 16:35:15] PHP Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 71 bytes) in /wp-includes/taxonomy.php on line 3720
What does that mean?