The certificate for this server is invalid…
Error message:
The certificate for this server is invalid. You might be connecting to a server that is pretending to be {yourschool}.passtab.com which could put your confidential information at risk. Would you like to connect to the server anyway?
For assistance with this error message please visit this article: Check System Status (iPad Online/Offline)
Western Australia (Fortigate)
If the school is a Western Australian school and is using Fortigate for their server this issue can occur even after the whitelisting has been implemented.
Make sure your iPad is connected via a Service Account and has a static/reserved IP address. It should also have Outbound access to everything to bypass the Fortigate server.