I've been having the same problem on my unlocked AT&T 1520.2 RM-940 that I brought over to T-Mobile. I never had installed Developer Preview on it. Did reinstalling DP allow Denim to download and install? I would get the 8018830f error after it downloaded the whole update and started installing it. I think it gets a small fraction into the install before giving "The unable was downloaded but couldn't be opened." Let me know if installing DP worked for you.
Yes, I had originally installed DP and uninstall it awhile back. When I got this error I didn't have it install. After installing DP and enable, I was able to complete my upgrade to Denim.