eReader authorization no longer working

Can someone help me please?

Your eReader is not authorzed to open this book

​Well actually it is...

​It has been working perfectly fine until recently

​I have factory reset and unistalled and reinstalled Adobe Digital Editions; I have deauthorized and reauthorized

​This is my son's eReader and this is the preferred way to borrow books from his school library

He owns a Kobo Aura H2O

​Please help

​Warm regards,

​Tracey