The reason is, that the content in the login window is shown within an <iframe>, and Find Elements will not operate into the <iframe>:

What you’ll have to do after opening the login window:
-
Get the
<iframe>using a Find Elements node (use e.g. an ID queryiframe_login, or XPath//*[@id='iframe_login']) -
Use the Frame node, check ‘WebElement’ and select the
<iframe>column from previous step -
Continue with your existing workflow
In case you need to ‘escape’ from the <iframe> later, you can use another Frame node with the option ‘Back to parent frame’ or ‘Default content’.
Does that help?