The form cannot be submitted to the web server either because your computer is offline

In SharePoint 2016, I have tried to submit a page for approval as shown below:

The approval workflow task form has been loaded properly.

But When I clicked on Start to initiate the workflow, I got the below warning:

The form cannot be submitted to the web server either because your computer is offline or because the host server is currently unavailable. If this problem persists, contact your network administrator

I tried to click on Try again more times with no avail!

Also, I had noticed that the internet explorer raised a risk warning “Only secure content is displayed” as shown below:

So I have followed the instructions mentioned at Fix Only Secure Content is displayed IE 11 In SharePoint to enable “Display Mixed Content” that solved the issue at this time.

The form cannot be submitted to the web server either because your computer is offline

Cause

This issue usually occurs if

• The SSL certificate is not installed on SharePoint Servers.
• The current page has been hosted within a secure HTTPS environment. Meanwhile, it embeds unsecured HTTP contents/scripts.
• If you have enabled SSL offloading, make sure that you have configured the Alternative Access Mapping Correctly.

Solution

1. As a temporary solution, try to use Google Chrome and enable Mixed content as mentioned at Only Secure Content is displayed.
2. As Permanent Solution,
   1. Install SSL Certificate on the SharePoint Servers, or
   2. Configure SSL offloading in SharePoint Farm.

Read more at This page is trying to load scripts from unauthenticated sources in SharePoint 2013