What is your PWD when you download the key file, in a terminal?
Is this location writeable by your (logged in) Linux user?
Are the contents of the downloaded Release.key file, the same as the online copy?
I've got the exact same problem for the last couple of weeks. I've tried reinstalling fresh latest Debian stable twice & got the same issue. It used to work fine on previous fairly recent installations. I tried giving it some time in case the file got changed but I've just re-downloaded it & it still looks the same. Is the Release.key file valid still? It just looks like a HTML file & I can't find "PGP" within it when searching. I'm no expert but the Release.key looks broken.
Just in case anyone is reviewing my last post - I was wrong. I had been following instructions for wine-staging. Their install instructions point to HTML page.