I certainly have cookies enabled and a second attempt to login works every time. Any ideas?
This was certainly happening pre 3.6.3, but I don't know how far back.
Are you using AD authentication over LDAPS connections? I got this error when LDAPS failed on the domain controller....maybe your issue is related?
Try this post on retrohack.com if you want to see how I fixed it.
I see this myself with Firefox 3.5, inconstantly. I believe my proxy is involved, probably cacheing an old answer, but I haven't looked into it.
The cookie detection routine sets an initial cookie on load -- perhaps there is something there blocking the checker? Can you clear all your cookies on the Splunk domain and try again?
What is your browser/platform? Also, what URI are you using to access the login page?