If this is affecting all of your engineers, then check that the server that hosts the mobile app is up and running, and that IIS is running.
If this is affecting only a single user, check that they can access the internet separately from the app.
If they can access the web without issue, then check the following:
Log into Access Administrator in order to update AMS users
Click Users
Double click on the user unable to log in
Edit their user name. Try a shorter user name, as a user name of 35 characters in length or over can cause this login issue. Try with their Firstname.last initial @ domain.com.
Click Save and Close
Now try and log in as the user, if it still fails immediately then contact us, please see the following Get support in Service Manager.
If it loads, then you'll be able to log in successfully, or if you have any further issues then please raise a case as above.