ANGIE has detected that this installer is already in use by someone else. This could be either a different person or even you using a different browser, computer or a different domain name to access the site. Please remember that http://example.com, https://example.com and http://www.example.com are three different domains as far as the restoration script can tell. This could also happen if you are trying to restore on a live site and a visitor accidentally accessed the restoration page before you.
public_html. This depends on your host. If unsure, ask your host.installer and go inside it.tmp and go inside it.storeagedata-b105e0e192aa61ed7bebb54e63215678.dat if such a file exists.storagedata-This page is a security precaution. When you are restoring your site, anyone visiting your site could see the restoration script. They could either copy private information from the installer pages or abuse the installer to point your site to an external database, under their own control, surreptitiously exfiltrating sensitive data from your site without you noticing it.
Here at Akeeba Ltd we are committed to protecting your security and privacy. That's our mission statement. One of the many ways we do this is locking down this installer when we have reasons to believe it might be abused to violate your security and privacy. Requiring you to delete files from your site is the only way we can positively prove that you are indeed in full control of the site, meaning it's safe to allow you to proceed with the restoration.
The best way to avoid random visitors from seeing the restoration page is to apply password protection. You can do that in the Configuration page of the backup software you used to create this backup archive. Find the field called ANGIE Password and enter a password. You will need to enter this password to unlock and use this installer when resstoring your site. People who don't know this password will not be able to see any privileged information contained in this installer.