For future generations: I also had this problem. I also noticed that I could log in if I set up a keyboard and display on the server. After much trial and error, I was eventually led by one of the Discord helpers [Thanks, jcofer555!] to trying a different browser, and that also worked. After clearing cookies and temp files in Firefox for Tower.local, <MyServer>.local, and all local IP addresses, I was able to gain access through the web portal on Firefox again.
So, anyone having similar issues... try hooking up a display and keyboard, try a different browser, and try clearing your cookies & temp files. Hope this is helpful!