righto,
had this problem a few times and know of other who have had it.
list of tryable solution:
1) you was on a server that restarted map or reset as you were joining, this then creates a blip in paradox/talewords cd keys and this error appears, one way of fixing this is if you can find the origional server you got this from and get them to fully restart the server (take it down put it back up) then retry joining.
2) you can delete a file within steam files called clientregistry.blob and Appupdatestats.blob - this for some will refresh the keys and re authorise the ones you had. its just in the main steam folder not in the mods or games just below all the files.
to do this stop steam - delete files- restart steam and it'll re-add the files and refresh them
3) go to the taleworlds forums and activate your steam CD manually on their database so it will take your steam one and your paradox key at the same time.
when i had this error none of this worked for me but worked for some. For me it was working the next day just had to leave it overnight for everything to fix itself.
regards Rackio