Hi otwcaravan,
Thank you for update.
Based on your reply, I understand the error message occurred when signing in to your MOP (Microsoft Online Portal) page.
This issue may be caused by following reasons:
1. Internet Explorer Settings.
You can refer to the following steps to narrow down the issue.
a. Clear all cache files of Internet Explorer, such as cookies and browsing data.
b. Open IE and click "Tools"-->"Internet Options"-->"Security"-->"Trusted Sites"-->click "sites" and add: https://*.sharepoint.com/ and https://*.microsoftonline.com/
c. Restart Internet Explorer, and try again.
d. Please make sure the Office 365 account ID and password are correct.
Moreover, I would like to suggest you try to log in to MOP via other web-browsers to see if it works.
2. Domain name may be incorrect
If you have verified your own domain and users have been using the verified domain, your login ID changes too. The "username@domain.onmicrosoft.com" will no longer work and you have to use the verified domain like "username@domain.com".
Thanks,
Allen Zhang