Forgive me, I have been traveling this weekend and with the Mfd9 update, I just caught your message.
The user agent field wouldn't be it. and certainly not with the url string in your picture.
My test was using my drive account and a test sheet. Note that there is a difference between a "Sharable link" and the Exported CSV link referenced in the browsers downloaded list.
Do you have the shareable link? Having this would allow, at least, ability to view in the browser?
(note: The sharable link is Not the correct link for Manifold, it would allow you only to view the sheet in the browser.)
If yes, can you regenerate the .CSV download? Does it download the file?
If yes, open up your browser's Downloads window showing recent download list. Find the downloaded file link, right button click on the link and click "Copy link address" from the context menu? the URL should begin with "https://docs.google.com/spreadsheet...."
This is the needed link, now copied to the clipboard.
A quick test outside of manifold: open notepad.exe
pull down the File / Open menu
in the window at the bottom will be the File name: edit box... Paste the link here.
next to this, insure that the "Text documents (*.txt)" is selected. Click the Open button.
This should if you have access open the CSV file inside notepad, naming the file "export".
If this works correctly, try the Manifold procedure:
Open Manifold v9. Pull down the menu, "File / Create / New Data Source..."
In the opening dialog box, Give the datasource a meaningful name.
Pull down the Type: box to "Web Server: csvserver"
Paste the correct link in the Source: box and then click the "Create Data Source" button at the bottom left.
The datasource should then show in the Project Pane. click to expand it.
An export table name should appear.
attached is the dialog screenshot.