i made some progress and thought I'd report it. I had an AHA! moment when I figured out the USERNAME is case sensitive. This may have been noted in the instructions somewhere (which I didn't read), but it is counterintuitive since this is not the way Windows works. So make sure you're using the right case. For your intermittent problem, I'm sure this is not the issue (I hope I never hit that issue). I also had an issue with a strong password (uses letters, numbers, and punctuation). When I reset it to a simpler password it worked, but another time, the strong password worked. So maybe this IS the intermittent problem you've had and it was the password reset itself that fixed me. I guess time will tell.
good luck and if you make any progress please report it here as there are many people having this issue on a number of DLink devices.