One thing that can be an 'invisible' factor is that if you use Windows Notepad and possibly other word processing programs on your key files, the CRLFs are not *nix compatible.
I generated the keys in Terminal on a Mac laptop and copy/pasted the public key directly from Terminal to the public keys tab of my user in the OMV GUI.
I used the procedure here to generate a key on my Linux Mint machine and send the public key to my OMV machine. I didn't use any portion of OMV's WebGUI to do this. It was all done from the remote machine.
I'll check it out.