SSH access - "Server refused our key" error

Hello all,

I am new to this forum, having owned a ReadyNAS NV+ since last october, and have an issue with SSH access.
Recently, my NV+ started refusing my authentication key out of the blue. This is what I did originally:

Running Radiator 4.1.7, I have enabled SSH and root SSH login.
Then, to increase security as I do on all my devices that support SSH, I enabled SSH for another user than root, then implemented private/public key authentication and tested it thoroughly using putty as my windows SSH client.
Once this worked fine, I disabled root login and password login to secure my network.
(I also installed and set up openVPN server support, but that is unrelated to my problem and still works anyway)

This worked fine for a while until a few days ago. Now when I try to log on the NAS (everything else being equal), I get a "Server refused our key" error.
For all I know the key file hasn't changed, and I haven't messed in CLI since setting everything up initially, but for good measure I tried with 5 different backups of the key file I had around (on my home PC, on my laptop, the ReadyNAS, a USB stick and my phone) with the same result.

I have to assume that the public key file got corrupted on the NV+ (that, in itself, is not a very comforting thought) and lost all CLI access to the unit as a result (root login, if enabled again, suffers the same limitation of course).

Is there a way other than a factory default to recover access to my NV+? It must be possible, for instance, to get an add-on similar to toggleSSH or EnableRootSSH that would re-enable password login, right? Is this forum the right place to ask for such an add-on?

Alternately is there any way to access the NV+ in a "console" mode (I guess not, but who knows what's possible with these units)?

thanks in advance
François