Invoice not shown or displayed on the page

Hello.

First of all check following things:

Try to use “Turn off compatibility mode.” option in Settings/Main tab.

Disable third-party plugins one by one to be sure that the issue is not related to any of them.

Try to switch theme to WordPress default(TwentyTwelve) to be sure that the issue is not related to the current theme.

And contact your host provider. It can be that they do not allow get parameters to be paste.

https://www.usabilitydynamics.com/product/wp-invoice/docs/empty-blank-invoice-page-or-just-wp-invoice-instead-of-invoice-in-wp-invoice-plugin

Regards.