Issue with Wordpress and Varnish breaking loadscript.php
I just stumbled on this issue today. You didn't post your configuration, but my guess is that you might be using querysort to normalize your query parameters.
I found that querysort really doesn't like the load-scripts.php URLs. In a lot of cases, it ends up truncating the URLs. If you're using it, I recommend either not using it or making a special condition for it like:
if (req.url !~ "load-scripts\.php") { set req.url = std.querysort(req.url);}
Further to Carl's answer I found that stopping querysort for the whole wp-admin helped with this error within other plugins.
if (req.url !~ "wp-admin") { set req.url = std.querysort(req.url);}