Verifique o valor atribuído a memory_limit em php.ini
Verifique também o sufixo de alocação de memória:
correto:
memory_limit = 128M
não está correto:
memory_limit = 128
Eu atualizei para o mysql 5.7 usando esta Atualização do MySQL 5.5 para MySQL 5,7
Desde então, recebemos 500 em todos os sites hospedados e o log de erros do apache2 contém
[Thu Jul 07 03:52:24.807409 2016] [core:notice] [pid 1624] AH00051: child pid 11486 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:32.819761 2016] [core:notice] [pid 1624] AH00051: child pid 11482 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:32.819853 2016] [core:notice] [pid 1624] AH00051: child pid 11489 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:35.824422 2016] [core:notice] [pid 1624] AH00051: child pid 11490 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:42.833487 2016] [core:notice] [pid 1624] AH00051: child pid 11493 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:46.839680 2016] [core:notice] [pid 1624] AH00051: child pid 11494 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:52.847820 2016] [core:notice] [pid 1624] AH00051: child pid 11495 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:52:56.852889 2016] [core:notice] [pid 1624] AH00051: child pid 11498 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:53:06.864664 2016] [core:notice] [pid 1624] AH00051: child pid 11500 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:53:07.867806 2016] [core:notice] [pid 1624] AH00051: child pid 11504 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:53:35.899735 2016] [core:notice] [pid 1624] AH00051: child pid 11506 exit signal Segmentation fault (11), possible coredump in /etc/apache2
[Thu Jul 07 03:53:39.906559 2016] [core:notice] [pid 1624] AH00051: child pid 11509 exit signal Segmentation fault (11), possible coredump in /etc/apache2
Configuração do servidor: Ubuntu 14.04, Mysql: 5.7, PHP 5.5.9, Apache 2.4.7