Error getting License. License Server Communication Problem:E_ADEPT_NO_TOKEN

I successfully purchased and downloaded two eBooks as acsm. files, and dragged them into Adobe Digital Editions (V4.5), and was able to read them. When I purchased a third book from the same vendor, I was able to download it, but could not drag it into Adobe Digital Editions. I get the following error message:

""Error getting License. License Server Communication Problem:E_ADEPT_NO_TOKEN"

The vendor tells me I have no tokens left and must contact Adobe.

Please can you explain what exactly the problem is, and how it can be resolved?