That's not a Moodle maintenance mode screen ... it's an operating system/apache (assuming apache) screen.
Check your servers web servers error logs for 413 errors.
Also suggest creating a phpinfo.php page and putting it at your servers document root ... which could be /home/customerlogin/public_html or /var/www/html/ or ??? depends.
The phpinfo.php page contains only the following:
<? phpinfo(); ?>
Hit the phpinfo.php page via your browser .... https://yoursite/phpinfo.php
That will provide a wealth of information on settings in new site.
Might need to increase some php values.
Might also check hosting providers forums/faqs/helpdesk blog, etc..
'spirit of sharing', Ken