just a side note -
"...the log file shows me that the DataModule was destroyed"
the UserSessionUnit is a DataModule.
why do you need to have both of them ?

I've successfully used a TTimer on a ServerController when installed as a windows service. Maybe a permissions issue as to what account you used when installing as a service?

Regards,
Mark

uses IW.Common.Threads;
 
  TMyTimedThread = class(TIWTimedThread)
  protected
    procedure DoExecute; override;
  public
    constructor Create(const AName: string; AInterval: Integer); override;
    destructor Destroy; override;
  end;
 
{ TMyTimedThread }
 
constructor TMyTimedThread.Create(const AName: string; AInterval: Integer);
begin
  inherited;
  // initialize your stuff here. Anything that is private to your thread
end;
 
destructor TMyTimedThread.Destroy;
begin
  // release anything created on Create herer
  inherited;
end;
 
procedure TMyTimedThread.DoExecute;
begin
   // do your work here
   // this method will execute in your thread context  once at each AInterval milliseconds
end;
 

var
  MyThread: TMyTimedThread;
begin
  MyThread := TMyTimedThread.Create('My Thread Name here', 10000); // will execute at 10 seconds interval
  MyThred.Start;  // (or Resume depending on your Delphi version)
end;
 
On ServerController.OnDestroy, call TMyTimedThead.Finish so you will signal the thread that it is terminated.
 

Kind regards