Jboss wildfly Web App is reponsive but cant render/show in the Browser in the list

We have a rather mysterious case. We have a web portal and it consists of 4 applications. These are displayed after login and the user can choose who wants to login. The problem here is that for weeks some users can not see 1 of these applications. When checking the code everything looks fine but the application is not listed or rendered with others. We changed the VM IPs a few weeks ago and have a lot of deployment to update the IPs. We did not program this web app ourselves and it was programmed 7 years ago! Does anyone have an idea what could be the reason that some users have this problem? We have tried to work with other browsers, clear cache etc.. But it doesnt Work