I uninstalled and tried to reinstall ArcGIS Server 10.1. On reinstallation I reached the create site option. I didn't have a useful site on the server, but the folders from a previous attempt were still on the server (c:\arcgisserver\directories c:\arcgisserver\config-store). When I attempted to create a new site it failed with "Unable to decrypt request parameters". Deleting the folders then creating the new site solved the issue.
I didn't see the error message on the forum so here is is with a quick solution.
I've seen it before. No quick solution. What I had to do is uninstall, then delete the arcgisserver folder (and subfolders), then reinstall. If anyone has a better solution, please post.
I solved it in the following way, the arcgis account that is created during the installation, I added it to the group of administrators, then I logged in with the arcgis account and I was able to successfully create the site. Really very simple