Error 2-5-6  

and connection time 11 sec

Hi @lucifer,
Is your graphics adapter Nvidia?
Do you maybe have Nvidia ShadowPlay/screen recording/instant replay running via Nvidia Geforce Experience?
If yes, please disable it before connecting spacedesk viewer app.

If problem still persist, could you please download the spacedesk diagnostic script here, then extract it.
https://www.dropbox.com/s/ilm5em35h0ygsfm/2020_spacedeskDiagnosticTool.zip?dl=0

Inside you will find spacedeskDiagnosticTool.cmd, just right-click it then run as administrator.
This script will start a debug view app to capture spacedesk debug traces.
On step 5 it will prompt you to reproduce the issue (by connecting the viewer app and reproduce the error code issue), make sure to do this before pressing any key to continue.
After the script is done, a folder will be created on the same folder as the script file which contains various log files that we need for analysis.
Please right-click it -> Send to -> Compressed (zipped) folder, then attach the *.zip file on your next reply.