Error connecting to dropbox

Hi,
I want to upgrade everything and wanted to backup first.
My google drive appears full (which I used before) so now I’m trying to connect to dropbox where I have loads more space.
When I try and update the updraft settings to dsave to dropbox I get an error message – “Dropbox/Curl error: invalid_request”.
Can you help?
It appears to backup okay if for remote storage I select “None”.
Where do thouse files get stored?
Would I still be able to use those if the site upgrade failed?
Thanks

https://ww.wp.xz.cn/plugins/updraftplus/