ReadyNAS 212: Set-Up of OneDrive bidirectional sync fails

Hi guys,
I´m trying to set up a sync between a directory in my local Netgear ReadyNAS 212 (firmware version 6.10.8 - the newest one) and MS OneDrive (free plan).

Here's what I´m doing to achieve that:

• log into my nas (DNS name "storage" in my local network)
• go to "Cloud"
• enable "Microsoft OneDrive"
• in the popup window, supply my MS credentials (https://login.microsoftonline.com)
• confirm that https://storage may receive my personal OneDrive data (if I do understand that correctly - see first attached screenshot):
  
• next, I try to add a session, but when browsing the OneDrive location, I get an error:
• I can ignore the error, see the empty root folder and add another folder (called "foo" for testing purposes):
• After that, I still receive the error that tells me I cannot list the direcrory from OneDrive:
• An interesting observation, however, might be that the folder "foo" has indeed been created in OneDrive (which I can see in the online version):
  

I also tried to use http://storage for redirect (not https, 'cause I suspected that the self-signed certificate might cause problems) but with no luck.

Funny to see that the same mechanism was working perfectly with the DropBox app (however I do not want to use DropBox for this as the rates would significantly higher in my use case).

What am I doing wrong?  Are there some prerequisites that I failed to fulfil (e.g. a paid plan of OneDrive)?

Thanks for any hint and best,
Benjamin