After disconnecting and attaching the content database with the message "Site error can not be found" in and in Chrome:
HTTP / 1.1 200 OK Server: Microsoft IIS / 8.5 Date: Mon 27.7.2015 06:56:52 GMT Connection: close
And in the log:
Category: Site Orphan Error: That's right UpgradeBlocking: False Message: Database [WSS_Content_DBONE] contains a site (Id = [798c2f51-4e9e-44bf-bfad-c774a7b563d0]Url = [/]) whose id is already assigned to another database (Id = [3637a130-6cf9-4932-8b9c-d85a7030c010]name = [WSS_Content_DBTWO]) in the sitemap. Consider deleting one of these sites, which have conflicting IDs. Remedy: The orphaned websites can lead to errors during the upgrade. Try to lose weight and re-insert the database containing the orphaned sites. If necessary, restart the upgrade.