minimwatch can't connect minimserver running on asustor

[jbrand]

Hello,
I have a problem to get a connection between Minimwatch on Windows 10 PC (64 Bit) and Minimserver running on an Asustor 6204T.
I have switched off the firewall on Windows side, but this makes no difference.
The change to a trial version changed neither.
The devices are connected by a TP-link switch.
I've installed JRE Version 8 Update 281 (Build 1.8.0_281-b09) 64 Bit.

I have changed the log to debug.

The only error I saw in the log was:
Startup complete
23:29:06.025 Thread-13: exception from in.readLine: java.io.IOException: Das Handle ist ungültig
23:29:16.621 AWT-EventQueue-0: TrayView: properties action
23:29:16.621 Thread-18: ServerControl: showProperties action

Minimserver is running fine. I get all files on a Marantz AV-Receiver SR6008

Any idea to solve the problem?

Best Regards
Jens