Please double-check all features that are enabled for the machine where IIS server is deployed (according to the Academy article here). Once done, please redeploy on-site application from scratch using clean binary files (which can be received from the support team at support@creatio.com).
It is as a must to use clean binary files (not those that can be provided by support in case you request database backup of some application to deploy it locally, but out-of-the-box files).
Also, please double-check the connection parameters to the database in connection. Since incorrect database connection parameters, the Strings config file can also lead to login issues.
And finally, please check if you haven't specified HTTPS as a redirect method for your application in IIS and if it was appropriately configured in the configuration files.
