Unable to connect to the database. Please contact your administrator.

I've setup a Devolutions server to use with Remote Desktop Manager.
Every step of the setup and every connection test reports no errors, I can reach the https://hostname/dvls/login page from both the Devolutions server and other servers in the network, but instead of a login prompt i get the following error "Unable to connect to the database. Please contact your administrator."

Same goes for the RDM Client:

Devolutions.RemoteDesktopManager.Business.TryGoOfflineException: Unable to reach server.
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.EnsureServerIsSupported()
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.LoginOAuth(String username)
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.Login(Boolean useCredentials)
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.Login()
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.get_WebClient()
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.<GetRepositories>b__145_0()
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.ExecuteAction[T](ExecuteActionDelegate`1 action, ExecuteActionLogMode logMode)
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.ExecuteActionMultipleData[T](ExecuteActionDelegate`1 action)
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.GetRepositories()
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSSecuritySubDataSource.GetRepositories(Boolean includeDefault)
   at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.GetDataSourceSettings()
   at Devolutions.RemoteDesktopManager.Managers.ConnectionManager.LoadDataSourceSettingsFromDataSource(ConnectionEngine engine)
   at Devolutions.RemoteDesktopManager.Managers.ConnectionManager.ValidatePoliciesPart1(ConnectionEngine engine, Boolean hasChanged)
   at Devolutions.RemoteDesktopManager.Managers.ConnectionManager.LoadConnectionsFromDataSource(ConnectionEngine& engine, Boolean hasChanged)
   at Devolutions.RemoteDesktopManager.Managers.ConnectionManager.LoadConnections(BaseConnectionDataSource dataSource)

This is a fresh setup.