An 'invalid token' error can occur if you download an eBook file (.acsm) before installing and authorising Adobe Digital Editions or another eBook reading program. 

The file becomes authorised to a non-existent account, and produces an 'invalid token' error when you try to open it.

If this error occurs, find the .acsm file in your downloads and delete it. 

Then install and open Adobe Digital Editions.

Go back to the eBook platform where you originally downloaded the file, and re-download it.

This should synchronise the new eBook file with your authorised Adobe Digital Editions account.