I have VLC 3.0.4 running on Debian 9.5 with LXDE
when I connect to my machine with Remote Desktop, I can open VLC, but the screen does not go to the HDMI-output.
when I open VLC on a screen that is connected with HDMI, VLC does not show, but is does run in the background.
as far as I can see, everything works over HDMI, except VLC

when I put the command vlc in Terminal, there is a complaint about "no suitable interface module" and also "interface "globalhotkeys,none"

have already looked around on the net, but no solution.
most discussions are very old and are dealing with starting VLC from console.
but I would like to open the gui from the program-menu or an icon on the desktop.

how can I solve this ??

would be nice to explain a bit better.
Is this a second monitor, a TV or ??? (connected to hdmi). Is this a laptop or desktop PC?
You mentioned remote desktop, so you need to have a client and a server host.

Exactly what did you try (on which host), what's happened....

ok, my machine is a Compulab Fitlet2, also know as Mint Mini Box.
normally they come with Mint, but I installed Debian 9.
the machine is connected with HDMI to a TV.
apart from that, there is ethernet and of course power supply.
no keyboard and no mouse.

I connect to my machine with SSH or RDP.
because RDP does not show the main screen rp0:, which is also shown over HDMI on my TV, I also use RealVNC.
when there is a connection with RealVNC, I can do anything and everything with my machine,
except starting the GUI of the media-player VLC.

this problem seems to have started with version 2.2.x of VLC.
but now version 3.0.x is current.
downgrading is not possible, because this causes all sorts of broken packages.

hopefully, somebody can help here.

I still do not understand how it was configured. Usually VNC only mirrors the original display, so you can see everything twice.
So again, how is it configured, what is the primary display (where the VNC server is running), how vlc started and displayed at all ....

that is correct.
once I have running what I want to see on the TV, I disconnect the VNC connection.

the primairy display is the TV, which is connected with a Hdmi cable.

vlc does not start, when I want to open it from the menu, or from an icon on the desktop.

or started, just no DISPLAY found, or whatever. You might want to start it from a console and/or check possible error messages.

the original display is showing on my TV.
I can open and use different progams, but not VLC, as the GUI of VLC does not open.

when I put the command VLC in Terminal, there is a complaint about "no suitable interface module" and also "interface "globalhotkeys,none"

did you check posts already like this: https://forum.videolan.org/viewtopic.php?t=60653 ?

yes, I have seen many posts like that,
but they are all very old and dealing with starting VLC from the command-line.
and I did try several suggestions from those posts, but the result is still the same, unfortunately

did you try -vvv ? was there any additional info?

what it comes down to is this: