It appears to be mostly a Mac problem… I’ve been having the same issue for the last year, unable to access Trip Planner using my MacBook laptop, iPad and iPhone. It brings up the sign-in page, begins to bring up my trips, and then goes back to the sign-in page. Normally, I use Safari or Firefox browsers on a Mac. Today I tried using Google Chrome on the Mac, and it worked. Surprise. The iPad and iPhone still do not work.
On the PC side, it has worked just fine… when I use a PC running Windows 10, it will access Trip Planner using Firefox, Chrome or Edge.
Sure would be nice if they would tell you they don't fully support Mac's