Hi, Riban!
Yes I was behind the curtains 
but very interested all the time. Finally found the moment!
Thank you very much for your fast response. Will try and back later.
Kind regards,
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Hi, again @riban!
After your answer, I changed the settings as suggested with no luck (after erasing data inserted on config.txt). 
For higher security, I did a clean zynthian install on micro-sd with balena etcher.
In the meanwhile, I tried a card with a raspbian default system and the monitor worked flawlessly at first attempt. 
A moment ago, I put the new ZYHTHIAN system card; on webconfig I selected Hardware/Display “Waveshare 7 HDMI+USB 1024x600”, getting the same ERROR.
It looks some kind of misconfiguration of monitor when working on ZYNTHIAN, or something else I cannot figure out… 
Show us your webconf dashboard.
Do you keep the Behringer UMC404HD connected when booting?
That was the problem for me. If you set a specific usb audio card you must keep that connected, otherwise you alwasy get an error.
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Ok, @Nicolaz
Actually is not connected. I started to think about as a cause of the problem… 
I will give a try and will come back.
Thank you very much! 
Well…
It seems than –that WAS– the problem!!!
It has booted up properly, without endless loops. Now that appears beeing quiet, I will give a try to my new setup. Will be in touch again… (I need to plaaaaay something 
)
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Zynthian uses jackd as it’s audio layer. Jackd needs a working soundcard to start. So you must configure jackd with the relevant soundcard and have that connected throughout the uptime of Zynthian.
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Fine. Understood.
Thank You.
Now I am running short of USB connectors. Should be fine using a USB hub attached to the PI 400?
It’s a weird resolution though, so needs some entries. I have a different monitor which is also HDMI+USB. If you have a look at the Waveshare HDMI+USB option in the Display hardware section of webconf it’s got the necessary stuff.
Hi, @jtode ,
I cannot fully understand your idea. When you recommend to look at the Waveshare HDMI+USB option in the Display hardware section of webconf, is to get the specifications of recommended displays?
If yes, I suppose that any display will need, at least an HDMI port + a USB port. The monitor bought, though it has a cable with 2 USB connectors, only one is needed to work. The 2 usb´s are only to work under Raspberries 3.
I am using it with only 1 attached, and the tactile is working fine .
The shortage comes from: 1 USB for UMC404hd; 1USB for mouse; 1 USB for monitor. End of ports. But… I need to connect a keyboard.
After typing this I am going to test a keyboard attached to the midi input of the UNMC404.
However, I should be interested in adding more ports, for other keyboards or accessories.
May exist any kind of hat connected to GPIO?
On the other hand, I want to thank all of you, for your fast and kind support.
I am living a fantastic experience in this community, until now, only read in the posts of other people. 
You can attach a USB hub to your Pi 400. I would recommend attaching the audio interface directly to the Pi and using the hub for peripherals like keyboard, mouse, touch, etc.
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Thank you @riban,
I need to look for a good one. Past experiences with USB hubs make me to be very careful.
I´ve been playing with a midi keyboard plugged to UMC404hx midi-in, and the experience was not specially pleasant as opposite as when played connected to USB directly. Some delay, lag, when playing. Not heavy but disgusting. I am a piano player and look for a good touch feeling. My first test (connected to my TV set) was really good.
On the other hand, I got working the screen BUT, not showing all the image (some parts were missing). Right now I am going to change settings with all the gear connected –of course!–
I will inform you about new achievements.
Hi, again,
After a new system reset I have found that, IF you connect audio card, screen, mouse, etc and then you set the first webconfig, then the screen works and fits perfect, with GENERIC HDMI display.
Thanks to all
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Does touchscreen work with GENERIC HDMI?
Yes!
Moreover, after setting it properly, the overall behaviour has gotten better, perhaps, even I appreciate less delay when playing.
Now it´s time to load new sounds and try.
I’ve always used the Generic HDMI display settings for 7" touchscreens.
The down side is if you have version 1.0 it won’t restart on reboot, so you have to powerdown.
Version 1.0’s seem to be a thing of the past generally, but always check.
