I have seen this error before and it seemed to be a Bug then. It looks like the issue is related to SiteRegisterController, so create the empty SiteRegisterControllerTest class and that will solve the problem
There is a known issue regarding this which was fixed earlier but try the above workaround.
Unable to create communities in certain orgs that have external ID or unique fields, validation rules, or triggers on the user object. The error appears "Oops, something went wrong; please refresh the page and try again" after trying to create a community.
1. Login as the admin of the org
2. Go to the Manage Communities Page
3. Click the New Community Button
4. Enter the name, description and URL
5. Click the Create button and the following appears "Oops, something went wrong; please refresh the page and try again."
Remove any external ID or unique fields, validation rules, or triggers on the user object. Otherwise create the empty class definitions (that is, the class is defined, but no methods or properties within them are needed) in the problematic orgs: