Did you ever figure this out. I know it's almost a year old but I'm having the exact same problem. I have a server with Acronis Backup, server 2012R2. I even installed sql 2012r2 express and shut down acronis so it would find the new instance. Credentials are good on both instances of SQL and I get the unaccepted credentials also. I also can not select the radio button that asks if I'd like to install sql 2012r2 express. It doesn't highlight when I click it. I'm out of options for now unless you found a fix.
I downloaded the iso and unzipped it with 7zip. I'm starting the program by running the autorun. I have all the prerequisites taken care of that it requires.
Any ideas?