Sadly I have tried contacting Magister Frost via PM and email, email was rejected and I have not heard back on the PM. Hopefully it is like you said just revamping the site blocking new users from joining.
If it's any consolation there really is not much activity on the Undercroft anymore anyway. If you're looking to converse with more Satanic minded individuals you would get more interaction through Facebook on the unofficial CoS group.
As Magister Nemo has argued before I agree that the discussions here on LttD are more intelligent and organized than the ones on FB. Still, the volume of participants on the FB group is tenfold. Moreover, you would be more in the loop on new CoS related books, merchandise, and apparel being offered as well as events. Such updates are simply not posted very often anymore on this message board. HERE
is the link to the unofficial group if you are interested. (You will have to request to be admitted to the group.)