This if often caused by memory limitations (script runs out of memory due to multiple plugins and complex theme installed on site).
To fix this memory limits need to be increased from the default of 40M. Usually 128M fixes most setups, but 256M can be used if more is required for certain operations. This can be done from wp-config.php :
define( 'WP_MEMORY_LIMIT', '128M');
A plugin like WP Config File Editor can be used.
That doesn’t work? Error can also be caused by bad plugin and/or theme code that generates memory usage loops.
Disabling plugins and switching to a standard theme will fix the issue in this case. Then plugins/theme can be reactivated one by one to identify which one/combination is causing the issue.
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