8.04 to 8.10 all pages return errors after upgrade

[divisionx]

Whoops\Exception\ErrorException: Maximum execution time of 300 seconds exceeded in .../vendor/whmcs/whmcs-foundation/lib/View/Template/CompatUtil.php:0
Stack trace:
#0 .../vendor/whmcs/whmcs-foundation/lib/Utility/Error/Run.php(0): WHMCS\Utility\Error\Run->handleError(1, 'Maximum executi...', '/home/xxxx...', 0)
#1 [internal function]: WHMCS\Utility\Error\Run->handleShutdown()
#2 {main}

Of course opened a ticket as this is a complete disaster. Replacing from backup.

[xyzulu]

You can probably just do a manual upgrade (not hard) instead of any automated one to resolve this.
See: Performing a Manual Update - WHMCS Documentation

Edited January 4, 2021 by xyzulu

[WHMCS Alex]

Hi,

This issue may occur when an order form template contains a theme.yaml file referencing itself as the parent theme.

To resolve it, ensure that any custom order form templates in place don't reference themselves as the parent theme inside the theme.yaml file.

This has also been reported to our development team as CORE-16051 so it's better handled by the core product in future.

I hope this helps.