I didn't get a choice when I uninstalled it but I did a search and deleted everything that had "opera" in it.
What I finally figured out was I have a program called "System Mechanic" that was blocking both Opera and Firefox from signing into Microsoft. I uninstalled it and now I can use both browsers again.
Thank you for your help.
Masking as Chrome made no difference, but your tips led me to discover the issue was with Opera tracking protection. After adding an exception for https://login.proofpoint.com/ the problem resolves. Hopefully, this helps someone else, as well. Thank you, immensely, for your shared wisdom!
It seems to be an extension issue; I turned them all off, as you suggested, and the sign in page came up. I turned them back on before signing in, and it doesn't go anywhere.
When I have more time, I'll figure out which extension is causing the problem.
Thanks for your help.
@helmut-meukel said in [Solved]Downloaded pages (save as... html) are not stored on disk:
The "HTML only" is the culprit.
That's working fine for me too fwiw. If the suggested filename when you save is really long, try just "test.html" to see if it makes any difference. Maybe the "HTML only" one truncates long filenames differently and causes the error.