Sonic.net

I recently used the Member Tools | Website Addons link to install WordPress at http://rfo.org/wp. The site itself works fine, but when I go to change a default page I get a JSON error:

Updating failed. Error message: The response is not a valid JSON response.

I have tried a number of things based on a web search for that error, but to no avail. And since I cannot see the error logs I am at a loss for how to proceed. Any hints?

Thanks,
--Dave