We have a web application (C# mvc) which loads about 10 javascripts files. While using an internal network about half of these files will return error 500/502 in Fiddler and the IIS log. After hitting ctrl-f5 al javascripts are loaded correctly. But when a user navigates to another page the same thing happens. This only happens when a user is reaching the website from the internal network. When browsing from an eternal network the web app works without problem.
We think it might have something to do with a proxy server but are unsure where to start looking.
After a lot of research it had to do with the network of our client.