We had a student who could not connect to the DiLL server when she opened her DiLL client. When she'd click on the dropdown to "Choose a Classroom", it would just say "Loading..." and would eventually error out with a message that it could not connect to the server.
This was only happening under her profile on her laptop. DiLL worked fine as admin or another AD account, so that definitely pointed to something that was loading or running just under her. Another clue was when pinging the DiLL server (fuji.ea1785.org) from her profile, the initial resolution was the external address for fuji - 204.108.251.37 instead of its internal address - 172.16.9.125.
It turned out that she was logged in with her AppleID and she had "Private Relay" turned on in iCloud. This caused network traffic to route through Apple's DNS server and interfered with her connection to the DiLL server through the client. Turning off Private Relay fixed the issue in DiLL.