This happened after a customer migrated their hosting and the issue was being unable to access PHP sessions.
Warning: session_start(): open(/var/lib/php/session/sess_3e0oms763m6mrqekdnnmodlhs3, O_RDWR) failed: Permission denied (13) in /register.php(202) : eval()'d code on line 1
Solution: (use the correct ownership/permissions)
chown apache.apache -R /var/lib/php
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