Hi Sharon, you're right - just testing this again today and I too cannot get the workaround to work. To recap at the beginning of 2023 it worked for us to get the user to login to Canvas THEN click on the Commons navigation button to have their computer authenticate to the Commons External Tool, then the link from 'Copy Resource Link' button will work.
However it looks like things have got a lot more sketchy.
https://commons.eu-central.canvaslms.com/resources/7de68639523d47ae8ff3d7e8728a79f3?shared - doesn't work - only shared within our institution. Brings up central Canvas.instructure.com login screen resolutely displays (like you, this has nothing to do with our insitutional server/accounts so a dead end)
https://commons.eu-central.canvaslms.com/resources/5c211e7cb8754ce98068676d14531034?shared - does work, shared publicly, but not logged in (so can't use but can see)
Not sure if any of this is to do with the localised hosting of Commons as you mention or the different sharing rights above, but the workaround used to work. It also means the advice from @ngarimella1 no longer holds true. It's a shame because this is a prime vehicle for sharing resources across faculty whilst maintaining instructure autonomy.
The response you got from support shows I think that they don't have a handle of this - they seem to be saying 'look at it this way: it doesn't work, but at least it doesn't work for everybody'. Doesn't say much for excellence in their products and services!🤣