On a particular web page (URL = https://www.myopenmath.com/course/testquestion.php?cid=47835&qsetid=279245) using Chrome, I can't invoke 1P to fill in my credentials. I tried shift-command-X, nothing happened. I tried clicking in the username box, got the prompt to enter shift-command-X, nothing happened. I also tried 1PX from the Chrome tool bar, and finally 1P via the menu item. Nothing worked.
BUT, I can go to a web page on the same site and invoke 1P without a problem (even on a page that doesn't have any credentials to fill in.
FWIW, this window opens up as a popup window from another URL at the same site, if that helps at all.
1Password Version: 7.2.5
Extension Version: 1PX 15.0.1
OS Version: 10.14.5
Sync Type: Not Provided