I have this problem on Vivaldi 5.2.2623.24 (Chrome 100), I saw that it was reported before, as you indicated, I disabled all other extensions, and the problem persists.
Apps, extensions and user scripts cannot be added from this website
Also, I don't have the mime flag.
The possible solution that occurs to me would be to put an iframe of vivaldi://extensions/ and be able to install the extension from there.