After a Drupal upgrade, I got this: Fatal error: Call to undefined function drupal_check_memory_limit() in full_path/modules/system/system.install on line 206.
This is mainly because the update was incomplete. You should update includes directory as well, after updating the core module files.
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However, I still have some errors in the error report section. Some modules are not being recognized. I will come with an update after I manage how to fix those errors too.