Exception: Serialization of 'Closure' is not allowed in serialize() (line 154 of dblog.module)

Problem

• When an exception is thrown during a entity storage controller save() operation, all you get is this fatal error:
  Error message

Exception: Serialization of 'Closure' is not allowed in serialize() (line 154 of \core\modules\dblog\dblog.module).

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• Whereas the dumping the entry that was supposed to be logged contains the actual error that may be this:
  SQLSTATE[42S22]: Column not found: 1054 Unknown column ...

Cause

• The $log_entry being passed to hook_watchdog() contains a 'backtrace' in its variables, which in turn contains the DrupalKernel/DI container and other classes and Closures that cannot be serialized.
• dblog_watchdog() performs this:
        'variables' => serialize($log_entry['variables']),