[Elder Raven]

I've been using Calibre for a very long time, and have had the same Kobo reader for several years. Until now, I have never had problems with either Calibre or the reader

The last couple of times I loaded books onto the reader and tried to open them I got a message saying the file couldn't be opened because it was protected by DRM. This is not the case.
When I reset the reader, those files had disappeared.
I tried again just now with two books I downloaded from Amazon. They were AZW, which I converted to Epub using Calibre, and there was no indication of any DRM. The books open fine in calibre, but once again I got the same error message on the reader.

This is extremely frustrating. Has anyone else had similar problems, or have any bright ideas for a fix?

Thanks in advance