I've noticed this also recently when setting up some server side schedules. I knew there were no issues with privileges as I was using a Full Access account. I found that I had to try a few times for this to work for some reason - i.e. error message the first time I clicked the button after entering the Account Name/Password but worked the 2nd/3rd time I tried.
Try a few times and see how you go.
I had noticed that same problem when we were initially setting up this server: you often had to hit the 'Next' button after entering the account credentials at least a second time, because the first time it would respond with an error (don't remember exactly what it was). But on that 2nd or 3rd attempt it would go through just fine. So, I have already done that. I have tried 8-10 times accompanied by mild cursing even - that didn't help either.
Well, it is working now, but don't really know what fixed it. The one event that seemed to open the flood gates of working-ness was when the admin console appeared to crash on me.
I was using the console trying to log in...getting failures.
I edited the security permissions in the file...failures.
I switched the account to a local one (using the same name, like that other thread had mentioned)....failures.
I went to review the logs to see if something was being entered there...Admin-console behaved weird: I got logged out once, it would sometimes not refresh, and then at one point I got an error about a bad gateway. I also got an email from the server with Error 724 in it: "Admin Server process is not responding".
After that (the process appeared to restart on its own), once I logged back in, I was able to use the externally authenticated account without further troubles. Go figure.