Hi Mike,
Upped memory to 256M via a new php.ini file and in wp-config.php. Attempts to do via .htaccess returned a 500 error. None of this fixed the issue.
Read up on object-cache.php and I ‘think’ this is leftover from a previous cache plugin I trialed, but did not keep.
As a temporary fix I’ve renamed the referenced file to object-cache.php.off and site appears to be running OK with no examples of the error.
Difficult to tell if there was some conflict with WPJM or not – so very many thanks for the response and suggested fixes.
