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First, please make sure that you have entered the correct Download ID as described in https://www.akeeba.com/documentation/akeeba-backup-joomla/update-dlid.html 

Then, go to your backup profile's configuration and find the Post-processing Engine row. Make sure the “OAuth2 Helper” is set to ‘Provided by Akeeba Ltd’. If it's not, change it and click on the Save button before following the next step.

Again in your backup profile's configuration page, find the Post-processing Engine row. Click on the “Authorisation – Start here” button to reconnect Akeeba Backup to OneDrive. You will need to copy the Access Token and Refresh Token when prompted. Then, click on Save & Close.

This error message means that the Access Token sent by Akeeba Backup is invalid. However, I had you just log into OneDrive which means that you should have a correct Access Token for the next hour.

Even if you waited longer than an hour after authenticating before taking the backup, it still doesn't quite make sense. We have the Refresh Token. If the Access Token is missing / invalid Akeeba Backup sends the Refresh Token and your Download ID to our server. If your Download ID –which I had you check– is valid our server will contact OneDrive with our Secret Key and your Refresh Token, getting back a new Access Token. This is sent back to your server which will now continue using it with OneDrive.

Now you understand why I had you take three steps which looked a bit unrelated to each other.

Assuming you did follow all three steps, the only deduction I can make is that you either pasted something wrong, or there's something wrong with your host (maybe a misconfigured transparent caching proxy). Since the server is OVH, we know it does not have a transparent caching proxy – or, at least, that was the case until last week.

This leaves me with something you did by accident. Maybe you pasted something wrong, or accidentally missed a step, or maybe took the actions against the wrong backup profile. Please check if this is the case. If not, reply back and I will ask you for connection information to your site to see what is going on.

I would like to update you on this. Another client gave us access to a site which was returning the same error message and we found out that the problems comes from Microsoft's side.

Without any prior warning, or documentation update, they have changed the way they expect us to perform the upload of backup archives in small chunks. This change appears to only affect a very small number of user accounts which is why there have been only three reports (including yours) and why the majority of our clients (and our three different OneDrive user accounts) have not been affected.

The good news is that we found a workaround and we're going to be making a new release of our backup software later today to address this issue.

Awesome! Thank you for the feedback and apologies for taking such a long time to figure out what is going on. Microsoft doing something pretty much opposite to what they documented was not on my bingo card :)