[1.5.1] Failed to open stream autoload.php

Does the file /vendor/autoload.php exist?
If not, try reinstalling the plugin please.

What’s your PHP version?

Please try changing __DIR__ to dirname( __FILE__ ) manually in autoload.php
Does that fix it?

Ok, I think your PHP version is the problem.

As a quick way to find out your PHP version, please add the following at the top of the autoload_real.php file:
die( phpinfo() );

Reloading your WordPress page, you should now see the PHP version at the top.

If you’re working on your live server and don’t have a development environment, please create a new file with a random name in the root directory of your WordPress installation and add this to it:
<?php die( phpinfo() ); ?>

Then point your browser to this new file: e.g. example.com/random-file-name.php

If you have access to the server error logs and feel comfortable debugging them, that would really help narrow it down too!