RevoSSL won't install on MODx 2.1

I’m trying to install RevoSSL. I have the package downloaded successfully, but when I try to actually install it, the license agreement (and all the other installation screens) are blank and installation fails. I have tried this on two different sites - 1 running 2.1.1 and the other running 2.1.2. I can install other addons on both sites with no trouble.