Boost + Domain Access + flushing cache
Saw your post on d.o.: https://www.drupal.org/project/domain/issues/1673650
The suggested fix looks good and should still run:
function MODULENAME_domain_cron($domain) {
  global $base_root;
  if (isset($domain['path'])) {
    $base_root = $domain['path'];
  }
  boost_cron();
}

The error you have there seems strange. The argument should be passed to the hook_domain_cron method ?!?
UPDATE: You still can try to make the argument optional:
function MODULENAME_domain_cron($domain=[])

But I would debug that to find out the real reason ..
How does your code look for mymodule_hook_domain_cron?