It looks like you are receiving a 500 internal server error, this is usually related to an incorrect apache configuration. Pease PM me your /etc/httpd/conf/httpd.conf file and your /etc/httpd/conf.d directory so that I can review the settings.
Thank you
Nagios XI Upgrade from 2014R2.7 to 5.2.3 Failed (PHP Error)
Re: Nagios XI Upgrade from 2014R2.7 to 5.2.3 Failed (PHP Err
Also, if you didn't install with the offline installer previously then you won't be able to upgrade offline.