So when you say it is working well through VPN and also on the browser, i think something is block it either any policy on the network level or anything installed on local machines as soon as it connect to VPN that local machines respect the policy of the network and allow the teams to load the app, could be the website or data source used in the app is directing to a site which is being blocked on the machines which are managed by any kind of Anti-virus or management tools
