Black display over otherwise good connection?

[PangolinGold]

Pocketbook Touch HD3, firmware version Dec '23
Raspberry pi 4B running normal Debian-based OS
Client app installed on PocketBook and configured to connection parameters

Symptom: when I connect the PocketBook the screen goes black - except twice it worked fine and gave me a perfect replica of the desktop, was able to zoom in, run wordgrinder in a window, etc. This morning, back to black. No command window open on the desktop so nothing black it could be zoomed in on.

When I Stop RemoteInk on the server the display on the PocketBook immediately reverts to normal.

Details: RemoteInk server starts, status running, and as I say I did get a successful display twice. The local hotspot on the Pi is working, the correct network parameters are configured on the client, and the connection starts. When it fails to connect (eg when I stopped the server or the wifi is off) I get a Connection Failed message on the PocketBook. So the issue is, I get a good connection, and the display is black.

Is this a known issue, and can anyone help me figure out how to get it working? As I said, it did display properly twice yesterday so I'm not sure what I'm doing or not doing wrong. Thanks!

[jinlaohu]

I've the same problem, test on Inkpad3 and touch HD3, both monitors gone black after connected.
Oh, I know the cause, must use X11 system.