CiviCRM cron job failing under Joomla 3.7
It is not a civi issue - it is a joomla issue that impacts other modules as well 
the fix is in j 3.71 
It is a 6 line patch and 1 line deletion 

I have tested it and it works if you want to patch it yourself 

https://github.com/joomla/joomla-cms/pull/15605