Internal server error on Devolutions server

Hello
We are testing the Devolutions server for our company. The installation of the Devolutions server went smooth, no problems so far. We can login to the web console of the DVLS, but as soon as we click on "Connections", I receive the following error message:
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at Devolutions.Server.ServerCacheManager.GetConnections(SessionContext context, Byte[] clientVersion) in c:\Dev\devolutions\Websites\Server\Common\Managers\ServerCacheManager.cs:line 207
at Devolutions.Server.ConnectionManager.GetConnectionsWithRoles(SessionContext context, Byte[] clientVersion) in c:\Dev\devolutions\Websites\Server\Common\Managers\ConnectionManager.cs:line 166
at Devolutions.Server.ConnectionNavigationManager.GetConnections(SessionContext context, String currentFolder) in c:\Dev\devolutions\Websites\Server\Common\Managers\ConnectionNavigationManager.cs:line 26
at Devolutions.Server.Controllers.APIControllers.v2.BackendApiController.GetPartialConnections(String currentFolder) in c:\Dev\devolutions\Websites\Server\Website\Controllers\APIControllers\BackendApiController.PartialConnection.cs:line 35
at lambda_method(Closure , Object , Object[] )
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass10.b__9(Object instance, Object[] methodParameters)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ActionFilterResult.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.AuthorizationFilterAttribute.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ExceptionFilterResult.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Web.Http.Controllers.ExceptionFilterResult.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Dispatcher.HttpControllerDispatcher.d__1.MoveNext()


If we try to connect to DVLS using RDM, we receive the following error message, as soon as we try to connect to the database:
System.Net.WebException: The remote server returned an error: (500) Internal Server Error.
at System.Net.HttpWebRequest.GetResponse()
at Devolutions.Server.ApiWrapper.RestClient.GetInfoDataFromBodyRequest[T,K](String apiUrl, T postObject)
at Devolutions.Server.ApiWrapper.RDMSWebClient.GetData(DataOptionsEntity options)
at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.ca672ee2e2ecd95333b49c186092d0277.c20e7a175c4cf686430ff4e87fb942e7b()
at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.c4cfebae48c715cba5241d6a671d47c8d[cb75a0a5d15a1ad8b8a8a5e57cca214a3](c31561c0d54939abd223ac7c0cf30e7e6`1 c6cc9ab02b2ea9ccd42c7c0497677bcc2, String ce54111a2f696be07240ffb02871e6867)
at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.c5f4f6c72c053c0cd60a62abeeaa43fdf[cb75a0a5d15a1ad8b8a8a5e57cca214a3](c31561c0d54939abd223ac7c0cf30e7e6`1 c6cc9ab02b2ea9ccd42c7c0497677bcc2)
at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSWebApiClient.GetData(String proxyConnectionCacheID, Byte[] version)
at Devolutions.RemoteDesktopManager.Business.DataSources.RDMSConnectionDataSource.GetDataSourceSettings()
at Devolutions.RemoteDesktopManager.Managers.ConnectionManager.c29fbd34504079a3525ca4c390d5d4074(BaseConnectionDataSource cf40b69955d088020784eb92737f0a505)


Any idea, what the problem could be? DVLS is running on a Windows Server 2016 and the SQL database on SQL 2014.
All the IIS requirements are installed.
Best regards
René