You cannot upload to a folder that belongs to a different user / Drive and has been shared with you. This kind of operation would require discovering which path in the tree leading to the file is a shared folder mountpoint, retrieve its Drive ID and modify the upload URL to point to the actual drive hosting the shared folder (and hope the permissions at the OneDrive level are correct). While a desktop application can afford to spend 15 to 30 seconds for the discovery phase this would be detrimental for web software running within tight execution time constraints like Akeeba Backup so we chose to not implement it. To make it clear: we didn't implement this feature because in most cases it would cause your web server to time out and the backup to fail. Since the problem has to do with the delay coming from using the OneDrive API and the timeout restrictions of your server there is no viable workaround we can implement. Therefore we chose not to ship a feature that would be broken due to no fault of our own.
However, if you create a folder in your OneDrive account and share it with someone else you don't have that problem. The upload takes place inside your own Drive and everything works fine.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!