Some users are reporting a failure to login with their AD credentials to an internally hosted DNN intranet. When accessing the site they get a popup asking for their username and password (rather than integrated logon occurring). If they enter their credentials it fails (three times) before displaying an error page. Across our 4000 users this is only happening to around 3-4 staff per day, and not always the same staff.

The failures are shown in the DNN event viewer but the details provided are scarce and of no use. I cannot see a matching failure event on the web server.

Does anyone have any pointers to how I can troubleshoot the issue further?