My machine has had a lot of different versions of the Notes client over time and somewhere during the different installs of the 8.5 beta the single sign on just stopped working.

When inspecting the event viewer in windows an entry states:

"Notes Single Logon will not function properly: No mapping between account names and security IDs was done."

I found nothing of value at notes.net but I did find a blog entry with a tip that works.

Just run regedit and modify:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\NetworkProvider\Order

Move "npnotes" in front of "WebClient".

Thanks to Glen over at thesalmonfarm.org for this helpful tip.