Transferring ebooks from public library to Tolino reader using own computer

Hi

I have a similar(ish) problem with transfer to my Kobo Clara HD from my Windows 10 laptop.  I can read the downloaded ebook from a public library on my laptop, never any problem.  I have the exact same Adobe ID authorising the laptop and the Kobo ereader (ADE itself tells me each authorisation and I have checked every character in each authorisation: identical).  I can even open the book (after an apparent transfer) from the ereader drive showing in the library (i.e. connected to the laptop) - but maybe this is just picking up the book from the laptop's copy.  But after an apparent transfer and disconnecting the laptop the ereader does an abrupt "importing..) and then goes blank.  The ebook is listed on the Kobo but is entirely black and when I try to open it the message is that the ereader is not authorised to be opened because I deauthorised it (when in fact I authorised it successfully according to the relevant messages).  In the past I've been through the reset and start from scratch (including my laptop in the process) and I'm not doing that again.  It's a complete stand-off. I've done what I'm supposed to do and one part of ADE says I have done so correctly and another that I haven't (though its reason is directly contradicted by looking at the authorisation information).  I've no problem in transferring public library books from laptop to ereader in Overdrive.  Any ideas please?