WordPress Error Cases

What common WordPress error messages mean, their common causes, and what to check. Error messages are listed in their original English.

• There has been a critical error on this website.“There has been a critical error on this website.” in WordPress: causes and what to check
• ERR_TOO_MANY_REDIRECTSERR_TOO_MANY_REDIRECTS on a WordPress site: causes and what to check
• Fatal error: Allowed memory size of … bytes exhausted“Allowed memory size of … bytes exhausted” in WordPress: causes and what to check
• 500 Internal Server Error500 Internal Server Error on a WordPress site: causes and what to check
• Updating failed. The response is not a valid JSON response.“Updating failed. The response is not a valid JSON response.” in WordPress: causes and what to check