E reader authorization on ADE

I have a Kobo e-reader that recently began showing the error message that my reader was not authorized for DRM to permit reading books borrowed from my public library after downloading the DRM file on ADE. In attempting to resolve the problem, I had uninstalled ADE and reinstalled it, deauthorized my reader and reauthorized it numerous times without success, and the last time I tried to reauthorize the reader, I got an error message that I could not authorize because I had reached the maximum number of computers/devices allowed per ID. As far as I know, I have only one PC and one e-reader authorized, and the maximum is supposed to be six. Why am I getting this erreor message?