Denal1, it seems to be the Mac and Safari combination that causes the problem.

I posted several times earlier in this thread about this problem, as have several others. A week or so ago I sent an email via delta.com and received a reply with a technical assistance number to call. The person with whom I spoke didn't seem aware of this problem with Mac and Safari. (I referred him to FlyerTalk; he had never heard of it) His only suggestion was to clean out my browser history, which I did to no avail. He said he would also do some more investigation and that I should try again in 24 hours. In 24 hours my login problem had not changed; upon login I still got a message saying my session had expired.

Soon after that failed attempt I had to make a reservation, which I did on my iPad, since I could still log in there. Once I'd made the reservation I found that I could access it on my MacBook by using the confirmation number. Then yesterday I made another random attempt to log into my account in the usual way (SM #, PIN, name) from my Mac and suddenly it worked! And today it still works!

I don't know whether there was something the technical assistance phone person did or whether accessing my reservation a few times with the confirmation number somehow reset my login process. I suspect the latter. Or some extra karma that came with my MSP-BOS upgrade at the 6-day window, which has been rare in recent months!