You may want to check in to your settings for AdSense or how you set up your hubpages account. If you set up your AdSense account with a different email address from your set up on Hubpages, there may be an error in that department. Other than that I can not think of anything that may cause that type of error.
I just joined HP about 2 weeks ago and only have 7 hubs and I have not had any problems with the AdSense feature on them.
I hope this helps out.